necessary

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Absolutely essential. See Synonyms at indispensable.
2. adj. Needed to achieve a certain result or effect; requisite: the necessary tools.
3. adj. Unavoidably determined by prior conditions or circumstances; inevitable: the necessary results of overindulgence.
4. adj. Logically inevitable.
5. adj. Required by obligation, compulsion, or convention: made the necessary apologies.
6. n. Something indispensable.

Century Dictionary and Cyclopedia

1. Such as must be; that cannot be otherwise. As an inference, evidently of such a form that every like inference from true premises will always yield a true conclusion, in every state of facts. In philosophy it is requisite to distinguish an irresistible inference, the force of which may be blindly felt, from a necessary one, which is seen to belong to a possible class of inferences, all true.
2. Such that it cannot be disregarded or omitted; indispensable; requisite; essential; needful; required: as, air is necessary to support animal life; food is necessary to nourish the body.
3. In law:
4. Requisite for reasonable convenience and facility or completeness in accomplishing the purpose intended: as, the land necessary for building a railroad.
5. Naturally and inseparably connected in the ordinary course: as, necessary consequences. Thus, the necessary consequences of a trespass, such as depreciation in value of a thing injured, or the suffering of a person injured, are general damages, and need not be pleaded; but loss of profits or medical expenses are not necessary consequences in the legal sense, and must be specially alleged.
6. Acting from compulsion or the absolute determination of causes: opposed to free. See free.
7. Synonyms Necessary, Essential, Requisite, Needful. The following remarks refer to the application of the words to ordinary practical affairs, not to philosophy. Necessary is so general a word that it covers all the others, and has the additional sense, which they do not have, of inevitable. Essential is an absolute word, noting that which is a part of the chief end of the action, or of every mode of bringing that end about. Requisite is less strong than essential, and needful is less strong still; yet each is strong and emphatic, applying to that which is imperatively needed. Needful generally applies to concrete, and often to temporary, things: as, knowledge of the countries visited is requisite, and even essential, to enjoyment of travel, but money is needful in order to be able to travel at all. Needful is often applied to that which must be supplied to produce or effect a perfect state or action.
8. n. Anything that is necessary or indispensable; that which cannot be disregarded or omitted: as, the necessaries of life.
9. n. A privy; a water-closet.

Wiktionary

1. adj. needed, required
2. n. archaic, UK bathroom, toilet, loo

GNU Webster's 1913

1. adj. Such as must be; impossible to be otherwise; not to be avoided; inevitable.
2. adj. Impossible to be otherwise, or to be dispensed with, without preventing the attainment of a desired result; indispensable; requisite; essential.
3. adj. Acting from necessity or compulsion; involuntary; -- opposed to free.
4. n. A thing that is necessary or indispensable to some purpose; something that one can not do without; a requisite; an essential; -- used chiefly in the plural.
5. n. A privy; a water-closet.
6. n. (Law) Such things, in respect to infants, lunatics, and married women, as are requisite for support suitable to station.

WordNet 3.0

1. n. anything indispensable
2. adj. unavoidably determined by prior circumstances
3. adj. absolutely essential