The Collaborative International Dictionary of English v.0.48:

Cognovit \Cog*no"vit\, n. [L., he has acknowledged.] (Law)
   An instrument in writing whereby a defendant in an action
   acknowledges a plaintiff's demand to be just. --Mozley & W.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

COGNOVIT, contr. leading. A written confession of an action by a defendant,
subscribed but not sealed, and authorizing the plaintiff to sign judgment
and issue execution, usually for a sum named.
     2. It is given after the action is brought to save expense.
     3. It differs from a warrant of attorney, which is given before the
commencement of any action, and is under seal. A cognovit actionem is an
acknowledgment and confession of the plaintiff's cause of action against the
defendant to be just and true. Vide 3 Ch. Pr. 664; 3 Bouv. Inst. n. 8299.