A facilinatrix (plural facilinatrices or facilinatrixes) is a woman who takes the assisting role in bondage and discipline, dominance and submission, or in group sex. She is usually focused on the pleasure of the Dominatrix or of both partners. She can facilitate position changes by providing oral/visual stimulation or physical assistance, and is generally included in the sexual experience by both of her partners. She may be a submissive of the dominatrix or she may be more of a trusted assistant/girlfriend/bridesmaid present to ensure a certain level of treatment for her Mistress. She is often not penetrated (though she may be) and acts where needed to heighten the sexual experience of both her partners.
A Facilinatrix is a woman in a threesome who is primarily facilitating rather than having intercourse.