In technical terms, acid is a substance that can react with a base to make a salt. In practical terms, acid is the stuff you want to keep off your hands, or any part of you, because it will eat away your skin.

Not all acids will eat skin away. The citric acid in oranges and other citrus fruits gives them their tasty tang. There is a hallucinogenic street drug called acid that eats away at the user’s sense of reality. Words sometimes have the qualities of acid. In acidic speech, words feel corrosive to the listener’s sensibilities. This one’s perhaps the most frequent and most destructive of all.

Definitions of acid
  1. noun
    any of various water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form a salt
    see moresee less
    show 107 types...
    hide 107 types...
    alcapton, alkapton, homogentisic acid
    an acid formed as an intermediate product of the metabolism of tyrosine and phenylalanine
    arsenic acid
    an acid formed from arsenic pentoxide
    cerotic acid, hexacosanoic acid
    a white solid fatty acid found in waxes (such as beeswax)
    chloric acid
    (HClO3) a strong unstable acid with an acrid odor found in chlorate salts
    chlorous acid
    (HClO2) a strongly oxidizing acid; known only in solution
    monobasic acid
    an acid containing only one replaceable hydrogen atom per molecule
    dibasic acid
    an acid containing two replaceable hydrogen atoms per molecule
    tribasic acid
    an acid containing three replaceable hydrogen atoms per molecule
    tetrabasic acid
    an acid containing four replaceable hydrogen atoms per molecule
    fulminic acid
    (CNOH) an unstable acid occurring mainly in the form of explosive salts and esters that is isomeric with cyanic acid
    gamma acid
    a crystalline acid used to make azo dyes
    hydriodic acid
    (HI) a colorless or yellow aqueous solution of hydrogen iodide
    hydrocyanic acid, prussic acid
    a solution of hydrogen cyanide in water; weak solutions are used in fumigating and in the synthesis of organic compounds
    hydroxy acid
    any acid that has hydroxyl groups in addition to the hydroxyl group in the acid itself
    hyponitrous acid
    an explosive white crystalline weak acid (H2N2O2)
    lysergic acid
    a crystalline acid often used in medical research; obtained from ergotic alkaloids
    manganic acid
    a dibasic acid (H2MnO4) found only in solution and in manganate salts
    2-methylpropenoic acid, methacrylic acid
    an unsaturated acid (C4H6O2) used to make resins and plastics
    selenic acid
    a strong acid (H2SeO4) analogous to sulfuric acid
    sulfonic acid, sulphonic acid
    an acid derived from sulphuric acid
    titanic acid
    a white weak acid that is a hydrated form of titanium dioxide
    perchloric acid
    a powerful oxidizing agent; forms perchlorates
    carboxylic acid
    an organic acid characterized by one or more carboxyl groups
    aminobenzoic acid
    a derivative of benzoic acid
    aqua fortis, nitric acid
    acid used especially in the production of fertilizers and explosives and rocket fuels
    nitrous acid
    an unstable inorganic acid known only in solution and as nitrite salts
    aqua regia, nitrohydrochloric acid
    a yellow fuming corrosive mixture of nitric and hydrochloric acid that dissolves metals (including gold)
    barbituric acid, malonylurea
    a white crystalline acid derived from pyrimidine; used in preparing barbiturate drugs
    Prevacid, lansoprazole
    antacid (trade name Prevacid) that suppresses acid secretion in the stomach
    boracic acid, boric acid
    any of various acids containing boron and oxygen
    boric acid, orthoboric acid
    a white or colorless slightly acid solid that is soluble in water and ethanol; used in the manufacture of glass and paper and adhesives and in detergents and as a flux in welding; also used as an antiseptic and food preservative
    bromic acid
    an unstable acid used as an oxidizing agent
    carbamic acid
    an acid that is known only by virtue of its salts (as ammonium carbamate) or its esters (as urethane)
    carbolic acid, hydroxybenzene, oxybenzene, phenol, phenylic acid
    a toxic white soluble crystalline acidic derivative of benzene; used in manufacturing and as a disinfectant and antiseptic; poisonous if taken internally
    carbonic acid
    a weak acid known only in solution; formed when carbon dioxide combines with water
    chromic acid
    an unstable acid known only in solution and as chromate salts
    citric acid
    a weak water-soluble acid found in many fruits (especially citrus fruits); used as a flavoring agent
    cyanamid, cyanamide
    a weak soluble dibasic acid (the parent acid of cyanamide salts)
    cyanic acid
    a colorless poisonous volatile liquid acid that hydrolyzes readily to ammonia and carbon dioxide
    cyanuric acid
    a trimer of cyanic acid
    ferricyanic acid
    a brown unstable acid formed from ferricyanide
    ferrocyanic acid
    a white unstable acid formed from ferrocyanide salts
    fluoroboric acid
    an acid of fluorine and boron
    fluosilicic acid, hydrofluosilicic acid
    an unstable poisonous corrosive acid known primarily in the form of its salts
    formic acid
    a colorless pungent fuming vesicatory liquid acid HCOOH found naturally in ants and many plants or made catalytically from carbon monoxide and steam; used in finishing textiles and paper and in the manufacture of insecticides and fumigants
    fumaric acid
    a colorless crystalline acid with a fruity taste; used in making polyester resins
    gallic acid
    a colorless crystalline acid obtained from tannin
    glyceric acid
    a syrupy acid obtained by oxidation of glycerol or glyceraldehyde
    glycolic acid, glycollic acid, hydroxyacetic acid
    a translucent crystalline compound found in sugar cane and sugar beets and unripe grapes
    hydrobromic acid
    an aqueous solution of hydrogen bromide that is a strong liquid acid
    chlorohydric acid, hydrochloric acid
    an aqueous solution of hydrogen chloride; a strongly corrosive acid
    hydrogen chloride
    a colorless corrosive gas (HCl)
    hydrofluoric acid
    a weak poisonous liquid acid; formed by solution of hydrogen fluoride in water
    hydroiodic acid
    an acid formed by aqueous solution of hydrogen iodide
    hypochlorous acid
    a weak unstable acid known only in solution and in its salts; used as a bleaching agent and as an oxidizing agent
    iodic acid
    a soluble crystalline acid; used as a reagent and disinfectant
    isocyanic acid
    an acid known only in the form of its esters
    itaconic acid
    a crystalline carboxylic acid; occurs in some fermentations of sugars
    maleic acid
    a colorless crystalline compound found in unripe fruit (such as apples or tomatoes or cherries) and used mainly to make polyester resins
    oxalacetic acid, oxaloacetic acid
    an acid formed by oxidation of maleic acid (as in metabolism of fats and carbohydrates)
    ethanedioic acid, oxalic acid
    a toxic colorless crystalline organic acid found in oxalis and other plants; used as a bleach and rust remover and in chemical analysis
    oxyacid, oxygen acid
    any acid that contains oxygen
    pantothen, pantothenic acid
    a vitamin of the vitamin B complex that performs an important role in the oxidation of fats and carbohydrates and certain amino acids; occurs in many foods
    PABA, para aminobenzoic acid
    a metabolic acid found in yeast and liver cells; used to make dyes and drugs and sun blockers
    pectic acid
    a complex acid that occurs in ripe fruit and some vegetables
    permanganic acid
    an unstable purple acid (HMnO4) known only in solution or of permanganate salts
    phthalic acid
    a colorless acid used to make dyes and perfumes
    picric acid
    a yellow toxic highly explosive strong acid; used in high explosives and as a dye and in chemical reactions
    pyruvic acid
    a colorless acid formed as an important intermediate in metabolism or fermentation
    dichromic acid
    the hypothetical acid (H2Cr2O7) from which dichromates are derived; known only in solution and in the form of dichromate salts
    silicic acid
    a jellylike substance (hydrated silica)
    sulfanilic acid, sulphanilic acid
    a crystalline acid made from aniline and used as a dye
    thiocyanic acid
    an unstable acid that can be obtained by distilling a thiocyanate salt
    toluic acid
    an isomeric acid derived from toluene
    triphosphoric acid
    an acid that is a partial anhydride of three molecules of phosphoric acid; known chiefly in the form of its salts and esters
    undecylenic acid
    an acid that is a component of perspiration
    uric acid
    a white tasteless odorless crystalline product of protein metabolism; found in the blood and urine
    oil of vitriol, sulfuric acid, sulphuric acid, vitriol
    (H2SO4) a highly corrosive acid made from sulfur dioxide; widely used in the chemical industry
    xanthic acid
    any of a class of unstable organic acids containing sulphur
    acetic acid, ethanoic acid
    a colorless pungent liquid widely used in manufacturing plastics and pharmaceuticals
    adipic acid, hexanedioic acid
    a carboxylic acid used in the manufacture of nylon
    hydroxybenzoic acid
    a crystalline derivative of benzoic acid
    hypophosphoric acid
    a crystalline tetrabasic acid (H4P2O6)
    hypophosphorous acid, orthophosphorous acid, phosphorous acid
    a clear or yellow monobasic acid (H3PO2)
    juniperic acid
    a crystalline acid found in the exudations of some conifers
    polyphosphoric acid
    a series of oxyacids of phosphorus
    mucic acid
    a solid acid (C6H10O8) found in milk or sugar
    octanedioic acid, suberic acid
    a dicarboxylic acid found in cork
    succinic acid
    a dicarboxylic acid (C4H6O4) active in metabolic processes
    saccharic acid
    a white dicarboxylic acid formed from oxidation of sugar or starch
    decanedioic acid, sebacic acid
    a dicarboxylic acid used to make resins
    sorbic acid
    a white crystalline carboxylic acid used as a preservative
    pentanoic acid, valeric acid
    a clear liquid carboxylic acid used in perfumes and drugs
    fatty acid
    any of a class of aliphatic monocarboxylic acids that form part of a lipid molecule and can be derived from fat by hydrolysis; fatty acids are simple molecules built around a series of carbon atoms linked together in a chain of 12 to 22 carbon atoms
    battery acid, electrolyte acid
    dilute sulfuric acid used in storage batteries
    benzoic acid
    a white crystalline solid occurring in many resins
    propanoic acid, propionic acid
    a liquid fatty acid found in milk and sweat and in fuel distillates
    lactic acid
    a clear odorless hygroscopic syrupy carboxylic acid found in sour milk and in many fruits
    muriatic acid
    a former name for hydrochloric acid
    periodic acid
    any acid of iodine that contains oxygen
    perboric acid
    a form of boric acid
    orthophosphoric acid, phosphoric acid
    an acid used in fertilizers and soaps: H3PO4
    acrylic acid, propenoic acid
    an unsaturated liquid carboxylic acid used in the manufacture of acrylic resins
    2-hydroxybenzoic acid, salicylic acid
    a white crystalline substance with a bitter aftertaste; used as a fungicide or in making aspirin or dyes or perfumes
    tartaric acid
    an acid found in many fruits; used in soft drinks and confectionery and baking powder
    tungstic acid
    an oxyacid of tungsten (often polymeric in nature) formed by neutralizing alkaline tungstate solutions
    vanadic acid, vanadium pentoxide
    any of various oxyacids of vanadium; known mostly in the form of its salts
    type of:
    chemical compound, compound
    (chemistry) a substance formed by chemical union of two or more elements or ingredients in definite proportion by weight
  2. adjective
    having the characteristics of an acid
    “an acid reaction”
    being or containing an acid; of a solution having an excess of hydrogen atoms (having a pH of less than 7)
  3. adjective
    being sour to the taste
    synonyms: acidic, acidulent, acidulous
    having a sharp biting taste
  4. adjective
    harsh or corrosive in tone
    “a barrage of acid comments”
    synonyms: acerb, acerbic, acrid, astringent, bitter, blistering, caustic, sulfurous, sulphurous, virulent, vitriolic
    disagreeable to the senses, to the mind, or feelings
  5. noun
    street name for lysergic acid diethylamide
Word Family

Test prep from the experts

Boost your test score with programs developed by’s experts.

  • Proven methods: Learn faster, remember longer with our scientific approach.
  • Personalized plan: We customize your experience to maximize your learning.
  • Strategic studying: Focus on the words that are most crucial for success.


  • Number of words: 500+
  • Duration: 8 weeks or less
  • Time: 1 hour / week


  • Number of words: 500+
  • Duration: 10 weeks or less
  • Time: 1 hour / week


  • Number of words: 700+
  • Duration: 10 weeks
  • Time: 1 hour / week