WordNet (r) 3.0 (2006):

bilateral contract
    n 1: a contract involving mutual promises (each party is both
         promisor and promisee)

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

BILATERAL CONTRACT, civil law. A contract in which both the contracting
parties are bound to fulfill obligations reciprocally towards each other;
Lec. Elem. Sec. 781; as a contract of sale, where one becomes bound to
deliver the, thing sold, and the other to pay the price of it. Vide
Contract; Synallagmatic contract.