consensual

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Of or expressing a consensus.
• adjective Law Existing or entered into by mutual consent without formalization by document or ceremony.
• adjective Involving the willing participation of both or all parties, especially in an illegal transaction or practice.
• adjective Of or relating to a reflexive response of one body structure following stimulation of another, such as the concurrent constriction of one pupil in response to light shined in the other.
• adjective Of or relating to involuntary movement of a body part accompanying voluntary movement of another.

from The Century Dictionary.

• Formed or existing by mere consent; depending upon consent or acquiescence: as, a consensual marriage.
• In physiology, of the nature of reflex action involving sensation but not volition.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Law) Existing, or made, by the mutual consent of two or more parties.
• adjective (Physiol.) Excited or caused by sensation, sympathy, or reflex action, and not by conscious volition.
• adjective (Law) a contract formed merely by consent, as a marriage contract.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective With permission, with consensus, without coercion.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective existing by consent