The Collaborative International Dictionary of English v.0.48:

Hypotheca \Hy`po*the"ca\, n. [L., fr. Gr. ? a thing subject to
   some obligation, fr. ? to put under, put down, pledge. See
   Hypothesis.] (Rom. Law)
   An obligation by which property of a debtor was made over to
   his creditor in security of his debt.
   [1913 Webster]

   Note: It differed from pledge in regard to possession of the
         property subject to the obligation; pledge requiring,
         simple hypotheca not requiring, possession of it by the
         creditor. The modern mortgage corresponds very closely
         with it. --Kent.
         [1913 Webster]