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necessary

[nes-uh-ser-ee]
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adjective
  1. being essential, indispensable, or requisite: a necessary part of the motor.
  2. happening or existing by necessity: a necessary change in our plans.
  3. acting or proceeding from compulsion or necessity; not free; involuntary: a necessary agent.
  4. Logic.
    1. (of a proposition) such that a denial of it involves a self-contradiction.
    2. (of an inference or argument) such that its conclusion cannot be false if its supporting premises are true.
    3. (of a condition) such that it must exist if a given event is to occur or a given thing is to exist.Compare sufficient(def 2).
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noun, plural nec·es·sar·ies.
  1. something necessary or required for a particular purpose; necessity.
  2. necessaries, Law. food, clothing, etc., required by a dependent or incompetent and varying with his or her social or economic position or that of the person upon whom he or she is dependent.
  3. Chiefly New England. a privy or toilet.
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Origin of necessary

1300–50; Middle English necessarie < Latin necessārius unavoidable, inevitable, needful, equivalent to necess(e) (neuter indeclinable adj.) unavoidable, necessary + -ārius -ary
Related formsnec·es·sar·i·ness, nounqua·si-nec·es·sar·y, adjective
Can be confusednecessary necessitous (see synonym study at the current entry)

Synonyms

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1. required, needed. Necessary, essential, indispensable, requisite indicate something vital for the fulfillment of a need. Necessary applies to something without which a condition cannot be fulfilled or to an inevitable consequence of certain events, conditions, etc.: Food is necessary to life. Multiplicity is a necessary result of division. Indispensable applies to something that cannot be done without or removed from the rest of a unitary condition: Food is indispensable to living things. He made himself indispensable as a companion. Something that is essential forms a vitally necessary condition of something: Air is essential to red-blooded animals. It is essential to understand the matter clearly. Requisite applies to what is thought necessary to fill out, complete, or perfect something: She had all the requisite qualifications for a position. 5. requirement, requisite, essential.

Antonyms

1. dispensable.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for necessary

necessary

adjective
  1. needed to achieve a certain desired effect or result; required
  2. resulting from necessity; inevitablethe necessary consequences of your action
  3. logic
    1. (of a statement, formula, etc) true under all interpretations or in all possible circumstances
    2. (of a proposition) determined to be true by its meaning, so that its denial would be self-contradictory
    3. (of a property) essential, so that without it its subject would not be the entity it is
    4. (of an inference) always yielding a true conclusion when its premises are true; valid
    5. (of a condition) entailed by the truth of some statement or the obtaining of some state of affairsCompare sufficient (def. 2)
  4. philosophy (in a nonlogical sense) expressing a law of nature, so that if it is in this sense necessary that all As are B, even although it is not contradictory to conceive of an A which is not B, we are licensed to infer that if something were an A it would have to be B
  5. rare compelled, as by necessity or law; not free
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noun
  1. the necessary informal the money required for a particular purpose
  2. do the necessary informal to do something that is necessary in a particular situation
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See also necessaries

Word Origin

C14: from Latin necessārius indispensable, from necesse unavoidable
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for necessary

adj.

late 14c. "needed, required, essential, indispensable," from Old French necessaire "necessary, urgent, compelling" (13c.), and directly from Latin necessarius "unavoidable, indispensable, necessary," from necesse "unavoidable, indispensable," originally "no backing away," from ne- "not" + cedere "to withdraw, go away, yield" (see cede). The root sense is of that from which there is no evasion, that which is inevitable. Necessary house "privy" is from c.1600. Necessary evil is from 1540s (the original reference was to "woman").

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n.

mid-14c., "needed, required, or useful things; the necessities of life; actions determined by right or law," perhaps from Old French necessaire (n.) "private parts, genitalia; lavatory," and directly from Latin necessarius (n.), in classical Latin "a relation, relative, kinsman; friend, client, patron;" see necessary (adj.).

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Online Etymology Dictionary, © 2010 Douglas Harper