Definition of necessary :
1. A privy; a water- closet.
2. 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.
3. Acting from necessity or compulsion; involuntary; - opposed to free; as, whether man is a necessary or a free agent is a question much discussed.
4. Impossible to be otherwise, or to be dispensed with, without preventing the attainment of a desired result; indispensable; requiste; essential.
5. Such as must be; impossible to be otherwise; not to be avoided; inevitable.
6. Such things, in respect to infants, lunatics, and married women, as are requisite for support suitable to station.
ingrained, specified, assured, principal, undeniable, wanted, prime, decisive, infallible, elementary, unessential, imminent, obligation, cardinal, momentous, necessity, expedient, fixed, urgent, exigent, unimportant, determinate, innate, paramount, needed, important, permanent, fundamental, constant, unavoidable, requirement, vital, needful, inherent, demand, basic, significant, chief, crucial, intrinsic, certain, quintessential
obligatory (part of speech: adjective)
required (part of speech: adjective)