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"