nec·es·sar·y 
(nĕsĭ-sĕr′ē)
(nĕsĭ-sĕr′ē)adj.
1. Needed or required: a contract complete with the necessary signatures; conditions necessary to life. See Synonyms at indispensable.
2.
a. Unavoidably determined by prior conditions or circumstances; inevitable: the necessary results of overindulgence.
b. Logically inevitable: a necessary conclusion.
3. Required by obligation, compulsion, or convention: made the necessary apologies.
n. pl. nec·es·sar·ies
Something indispensable.
[Middle English necessarie, from Old French necessaire, from Latin necessārius, from necesse; see ked- in the Appendix of Indo-European roots.]