being essential, indispensable, or requisite: a necessary part of the motor.
happening or existing by necessity: a necessary change in our plans.
acting or proceeding from compulsion or necessity; not free; involuntary: a necessary agent.
(of a proposition) such that a denial of it involves a self-contradiction.
(of an inference or argument) such that its conclusion cannot be false if its supporting premises are true.
(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 necessaries.
something necessary or requisite; necessity.
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.
Chiefly New England. a privy or toilet.

1300–50; Middle English necessarie < Latin necessārius unavoidable, inevitable, needful, equivalent to necess(e) (neuter indeclinable adj.) unavoidable, necessary + -ārius -ary

necessariness, noun
quasi-necessary, adjective

necessary, necessitous (see synonym study at the current entry).

1. required, needed. Necessary, essential, indispensable, requisite indicate something vital for the fulfillment of a need. Necessary applies to that 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. That which 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.

1. dispensable. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
Cite This Source Link To necessary
World English Dictionary
necessary (ˈnɛsɪsərɪ)
1.  needed to achieve a certain desired effect or result; required
2.  resulting from necessity; inevitable: the necessary consequences of your action
3.  logic
 a.  (of a statement, formula, etc) true under all interpretations or in all possible circumstances
 b.  (of a proposition) determined to be true by its meaning, so that its denial would be self-contradictory
 c.  (of a property) essential, so that without it its subject would not be the entity it is
 d.  (of an inference) always yielding a true conclusion when its premises are true; valid
 e.  Compare sufficient (of a condition) entailed by the truth of some statement or the obtaining of some state of affairs
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
6.  informal the necessary the money required for a particular purpose
7.  informal do the necessary to do something that is necessary in a particular situation
[C14: from Latin necessārius indispensable, from necesse unavoidable]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

mid-14c. (n.), late 14c. (adj.), from L. necessarius, from necesse "unavoidable, indispensable," originally "no backing away," from ne- "not" + cedere "to withdraw, go away, yield" (see cede). Necessary house "privy" is from c.1600. Related: Necessarily.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
You've got to feel fiercely loyal to your college to put in the necessary hours
  and endure the inevitable setbacks.
Unless your soil is very sandy, it's not necessary to add organic amendments.
Technicians made the necessary changes, and the experiment went forward.
Is it ever necessary for one squad to go assist another.
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