The Collaborative International Dictionary of English v.0.48:

Unilateral \U`ni*lat"er*al\, a. [Uni- + lateral: cf. F.
   unilat['e]ral.]
   1. Being on one side only; affecting but one side; one-sided.
      [1913 Webster]

   2. (Biol.) Pertaining to one side; one-sided; as, a
      unilateral raceme, in which the flowers grow only on one
      side of a common axis, or are all turned to one side.
      [1913 Webster]

   Unilateral contract (Law), a contract or engagement
      requiring future action only by one party.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unilateral contract
    n 1: a one-sided agreement whereby you promise to do (or refrain
         from doing) something in return for a performance (not a
         promise)

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

UNILATERAL CONTRACT, civil law. When the party to whom an engagement is
made, makes no express agreement on his part, the contract is called
unilateral, even in cases where the law attaches certain obligations to his
acceptance. Civ. Code of Lo. art. 1758. Code Nap. 1103. A loan of money, and
a loan for use, are of this kind. Poth. Obl. part 1, c. 1, s. 1, art. 2;
Lee. Elemen. Sec. 781.