People who are really good at math can handle complex math problems. The rest of us are all set with simple stuff like the times tables. Use complex to refer to something that is not simple.

Something complex has two or more related parts, or is not simple in its structure. As a noun, a complex is a group of units, often a group of buildings. The adjective is stressed on the second syllable, but the noun is stressed on the first syllable. Complex is from Latin complexus, from complecti "to encircle or embrace."

Definitions of complex
  1. adjective
    complicated in structure; consisting of interconnected parts
    “a complex set of variations based on a simple folk melody”
    “a complex mass of diverse laws and customs”
    difficult, hard
    not easy; requiring great physical or mental effort to accomplish or comprehend or endure
    composed of more than one part
    analyzable, decomposable
    capable of being partitioned
    Byzantine, convoluted, involved, knotty, tangled, tortuous
    highly complex or intricate and occasionally devious
    colonial, compound
    composed of many distinct individuals united to form a whole or colony
    difficult to analyze or understand
    consisting of separate interconnected parts
    consisting of two or more substances or ingredients or elements or parts
    complex and ingenious in design or function
    extremely intricate; usually in phrase `Gordian knot'
    interlacing, interlinking, interlocking, interwoven
    linked or locked closely together as by dovetailing
    having many complexly arranged elements; elaborate
    labyrinthian, labyrinthine, mazy
    resembling a labyrinth in form or complexity
    involving or depending on several factors or causes (especially pertaining to a condition or disease resulting from the interaction of many genes)
    having many parts or aspects
    becoming more intricate or complex
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    having few parts; not complex or complicated or involved
    posing no difficulty; requiring little effort
    not elaborate or elaborated; simple
    simple, unsubdivided
    (botany) of leaf shapes; of leaves having no divisions or subdivisions
    having only one part or element
    characterized by extreme and often misleading simplicity
    unanalyzable, undecomposable
    representing the furthest possible extent of analysis or division into parts
    uncomplicated, unsophisticated
    lacking complexity
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  2. noun
    a conceptual whole made up of complicated and related parts
    “the complex of shopping malls, houses, and roads created a new town”
    synonyms: composite
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    a composite of mixed origin
    a complex of concurrent things
    type of:
    all of something including all its component elements or parts
  3. noun
    a whole structure (as a building) made up of interconnected or related structures
    synonyms: building complex
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    Dartmouth College
    a college in New Hampshire
    United States Mint
    the mint that manufactures and distributes United States coins for circulation through Federal Reserve Banks; processes gold and silver bullion
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    a complex of buildings in which an institution of higher education is housed
    industrial plant, plant, works
    buildings for carrying on industrial labor
    ribbon development
    building complex in a continuous row along a road
    bottling plant
    a plant where beverages are put into bottles with caps
    a plant where beer is brewed by fermentation
    distillery, still
    a plant and works where alcoholic drinks are made by distillation
    factory, manufactory, manufacturing plant, mill
    a plant consisting of one or more buildings with facilities for manufacturing
    a plant where money is coined by authority of the government
    packing plant, packinghouse
    a plant where livestock are slaughtered and processed and packed as meat products
    recycling plant
    a plant for reprocessing used or abandoned materials
    an industrial plant for purifying a crude substance
    a plant where salt is produced commercially
    disposal plant, sewage disposal plant
    a plant for disposing of sewage
    smelter, smeltery
    an industrial plant for smelting
    type of:
    construction, structure
    a thing constructed; a complex entity constructed of many parts
  4. noun
    a compound described in terms of the central atom to which other atoms are bound or coordinated
    synonyms: coordination compound
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    a complex consisting of an organic base in association with hydrogen chloride
    type of:
    chemical compound, compound
    (chemistry) a substance formed by chemical union of two or more elements or ingredients in definite proportion by weight
  5. noun
    (psychoanalysis) a combination of emotions and impulses that have been rejected from awareness but still influence a person's behavior
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    Oedipal complex, Oedipus complex
    a complex of males; desire to possess the mother sexually and to exclude the father; said to be a source of personality disorders if unresolved
    Electra complex
    a complex of females; sexual attraction to the father
    inferiority complex
    a sense of personal inferiority arising from conflict between the desire to be noticed and the fear of being humiliated
    type of:
    the experiencing of affective and emotional states
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