The Collaborative International Dictionary of English v.0.48:

Requisite \Req"ui*site\, a. [L. requisitus, p. p. requirere;
   pref. re- re- + quaerere to ask. See Require.]
   Required by the nature of things, or by circumstances; so
   needful that it can not be dispensed with; necessary;
   indispensable.
   [1913 Webster]

         All truth requisite for men to know.     --Milton.
   [1913 Webster]

   Syn: Necessary; needful; indispensable; essential.
        [1913 Webster] -- Req"ui*site*ly, adv. --
        Req"ui*site*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

requisiteness
    n 1: the state of being absolutely required