What is "hypotheca"

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.

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.

n. (context microbiology planktology English) The lower or posterior half of the theca of a thecate protist such as a diatom or dinoflagellate