A Facilitator is a person who has the role of helping participants to learn in an experiential group.  The facilitator will normally be formally appointed to this role by whatever organization is sponsoring the group, and the group members will voluntarily accept the facilitator in this role.

By Experiential Group, we mean one in which learning takes place through an active and aware involvement of the whole person - as a spiritually, energetically and physically endowed being encompassing feeling and emotion, intuition and imaging, reflection and discrimination, intention and action (Heron, 1992). This potentially covers a wide spectrum: traditional therapy groups, sensitivity training groups, encounter groups, personal development groups in a particular mode (such as psychodrama, co-counselling, bio-energetics, primal, Gestalt, transpersonal, etc.), interpersonal skills training groups for personal or professional development, management training groups, social action training groups. The spectrum also extends into all contexts where learning is rooted in self-direction and whole personhood.

These definitions have been taken from John Heron's book "The Complete Facilitator's Handbook" (Heron 1999).  You can find out more about it or order it here.