To decay means to rot, decompose, break down. Our bodies—anything organic—will decay after death. Broken sidewalks, potholes, graffiti are all signs of urban decay. Tooth decay is something to avoid.

Decay can also mean decline. Anyone, even a great athlete, must exercise regularly to maintain muscle tone—as soon as you stop, the muscles begin to decay. If the strength of the cable signal coming into your house grows weaker over time, you can complain that it has decayed.

Definitions of decay
  1. noun
    the organic phenomenon of rotting
    synonyms: decomposition
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    type of:
    organic phenomenon
    (biology) a natural phenomenon involving living plants and animals
  2. noun
    an inferior state resulting from the process of decaying
    “the corpse was in an advanced state of decay
    “the house had fallen into a serious state of decay and disrepair”
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    putrefaction, rot
    a state of decay usually accompanied by an offensive odor
    decomposition, disintegration
    a decayed state
    deterioration, impairment
    a symptom of reduced quality or strength
    the state of being rancid; having a rancid scent or flavor (as of old cooking oil)
    the disintegration of social norms governing behavior and thought and social relationships
    corruption, putrescence, putridness, rottenness
    a state of progressive putrefaction
    a state of deterioration in metals caused by oxidation or chemical action
    desolation, devastation
    the state of being decayed or destroyed
    decrepitude, dilapidation
    a state of deterioration due to old age or long use
    impairment resulting from long use
    type of:
    a condition of damage or decay
  3. noun
    the process of gradually becoming inferior
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    caries, cavity, dental caries, tooth decay
    soft decayed area in a tooth; progressive decay can lead to the death of a tooth
    decay of matter (as by rot or oxidation)
    decomposition, putrefaction, rot, rotting
    (biology) the process of decay caused by bacterial or fungal action
    dilapidation, ruin
    the process of becoming dilapidated
    spoilage, spoiling
    the process of becoming spoiled
    mildew, mold, mould
    the process of becoming mildewed
    the process of becoming sour
    type of:
    action, activity, natural action, natural process
    a process existing in or produced by nature (rather than by the intent of human beings)
  4. noun
    a gradual decrease; as of stored charge or current
    synonyms: decline
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    exponential decay, exponential return
    a decrease that follows an exponential function
    relaxation, relaxation behavior
    (physics) the exponential return of a system to equilibrium after a disturbance
    type of:
    decrease, decrement
    a process of becoming smaller or shorter
  5. noun
    the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation
    synonyms: disintegration, radioactive decay
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    alpha decay
    radioactive decay of an atomic nucleus that is accompanied by the emission of an alpha particle
    beta decay
    radioactive decay of an atomic nucleus that is accompanied by the emission of a beta particle
    type of:
    nuclear reaction
    (physics) a process that alters the energy or structure or composition of atomic nuclei
  6. verb
    undergo decay or decomposition
    “The body started to decay and needed to be cremated”
    synonyms: degrade
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    decompose, molder, moulder, rot
    break down
    go bad, spoil
    become unfit for consumption or use
    fester, putrefy
    become putrid; decay with an offensive smell
    melt away in the process of decay
    break down naturally through the action of biological agents
    suspend (meat) in order to get a gamey taste
    become rotten
    go bad or sour
    type of:
    undergo a change; become different in essence; losing one's or its original nature
  7. verb
    fall into decay or ruin
    “The unoccupied house started to decay
    synonyms: crumble, dilapidate
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    become worse or disintegrate
    corrode, rust
    become destroyed by water, air, or a corrosive such as an acid
    change under the action or influence of the weather
    eat at, erode, gnaw, gnaw at, wear away
    become ground down or deteriorate
    droop, wilt
    become limp
    fall into ruin
    break, bust, fall apart, wear, wear out
    go to pieces
    go to pot, go to the dogs
    become ruined
    eat away, fret
    wear away or erode
    wear, wear down, wear off, wear out, wear thin
    deteriorate through use or stress
    fray, frazzle
    wear away by rubbing
    type of:
    undergo a change; become different in essence; losing one's or its original nature
  8. verb
    lose a stored charge, magnetic flux, or current
    synonyms: decompose, disintegrate
    cause to undergo fission or lose particles
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    type of:
    change integrity
    change in physical make-up
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