The Collaborative International Dictionary of English v.0.48:

Consensual \Con*sen"su*al\, a. [See Consent, v. i., and cf.
   Sensual.]
   1. (Law) Existing, or made, by the mutual consent of two or
      more parties.
      [1913 Webster]

   2. (Physiol.) Excited or caused by sensation, sympathy, or
      reflex action, and not by conscious volition; as,
      consensual motions.
      [1913 Webster]

   Consensual contract (Law), a contract formed merely by
      consent, as a marriage contract.
      [1913 Webster]

WordNet (r) 3.0 (2006):

consensual
    adj 1: existing by consent; "a consensual contract"

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

CONSENSUAL, civil law. This word is applied to designate one species of
contract known in the civil laws; these contracts derive their name from the
consent of the parties which is required in their formation, as they cannot
exist without such consent.
     2. The contract of sale, among the civilians, is an example of a
consensual contract, because the moment there is an agreement between the
seller and the buyer as to the thing and the price, the vendor and the
purchaser have reciprocal actions On the contrary, on a loan, there is no
action by the lender or borrower, although there may have been consent,
until the thing is delivered or the money counted. This is a real contract
in the sense of the civil law. Lec. El. Dr: Rom. Sec. 895; Poth. Ob. pt. 1,
c. 1, s. 1, art. 2; 1 Bell's Com. (5th ed.) 435. Vide Contract.