Necessary a. : Such as must be; impossible to be otherwise; not to be avoided; inevitable.

Necessary a. : Impossible to be otherwise, or to be dispensed with, without preventing the attainment of a desired result; indispensable; requiste; essential.

Necessary a. : Acting from necessity or compulsion; involuntary; -- opposed to free; as, whether man is a necessary or a free agent is a question much discussed.

Necessary 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; as, the necessaries of life.

Necessary n. : A privy; a water-closet.

Necessary n. : Such things, in respect to infants, lunatics, and married women, as are requisite for support suitable to station.

Example:
How Necessary is it to have your own sprinkler system in a single family house?

Taxonomy
Dictionary, Synonyms, adjective, compulsion, Food, word origin, nes, nec 780, dependent, privy, necessarie, necesse, Houghton Mifflin Company, free online English dictionary, dictionary results, pronunciation, free online dictionary, definitions.

Defination from the web:
Necessary - Definition of Necessary at Dictionary.com a free online dictionary with pronunciation, synonyms, and translation of Necessary. ... Necessary, essential, indispensable, requisite indicate something vital for the ... Multiplicity is a necessary result of division. ...