Dictionary .com says.... 1. formed or existing merely by consent: a consensual transaction. -or- 2.Physiology. (of an action) involuntarily correlative with a voluntary action, as the contraction of the iris when the eye is opened.
By choice, in agreement. When an act is consensual BOTH or all parties involved are agreeing to it freely without pressure or duress in sound mind. Legally if a person is drunk or stoned they are not 'in sound mind' and their choices and actions (sexual esp) may be considered NON-consentual for example and the other person could be charged with assault or rape after the fact.