The Collaborative International Dictionary of English v.0.48:

Declarant \De*clar"ant\, n. [Cf. F. d['e]clarant, p. pr. of
   d['e]clarer.] (Law)
   One who declares. --Abbott.
   [1913 Webster]

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

DECLARANT. One who makes a declaration. Vide Declarationis.