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)

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

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.