necessary

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).

noun, plural nec·es·sar·ies.

5. something necessary or required for a particular purpose; necessity.
6. 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.
7. Chiefly New England. a privy or toilet.

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

noun

6. the necessary informal the money required for a particular purpose
7. do the necessary informal to do something that is necessary in a particular situation

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”). 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.).