The Collaborative International Dictionary of English v.0.48:

Posit \Pos"it\, v. t. [imp. & p. p. Posited; p. pr. & vb. n.
   Positing.] [L. ponere, positum, to place. See Position.]
   1. To dispose or set firmly or fixedly; to place or dispose
      in relation to other objects. --Sir M. Hale.
      [1913 Webster]

   2. (Logic) To assume as real or conceded; as, to posit a
      principle. --Sir W. Hamilton.
      [1913 Webster]