Source : Webster's Revised Unabridged Dictionary (1913)

Assumpsit \As*sump"sit\ (?; 215), n. [L., he undertook, pret. of
   L. assumere. See {Assume}.] (Law)
   (a) A promise or undertaking, founded on a consideration.
       This promise may be oral or in writing not under seal. It
       may be express or implied.
   (b) An action to recover damages for a breach or
       nonperformance of a contract or promise, express or
       implied, oral or in writing not under seal. Common or
       indebitatus assumpsit is brought for the most part on an
       implied promise. Special assumpsit is founded on an
       express promise or undertaking. --Wharton.