Merriam-Webster's Collegiate® Dictionary, 11th Edition
NECESSARY
I. adjective see: no
Date: 14th century
1. of an inevitable nature ; inescapable , b. logically unavoidable, that cannot be denied without contradiction, determined or produced by the previous condition of things, compulsory , absolutely needed ; required,
II. noun (plural -saries)
Date: 14th century
an indispensable item ; essential