A tendon sheath is a layer of membrane around a tendon. It permits the tendon to move. It has two layers: Fibroma of the tendon sheath has been described. ...
<anatomy> A membranous sleeve which envelops the tendon and creates a lubricated low-friction environment for easy movement. ... (13 Jan 1998) ... 
a lubricated fibrous or synovial layer of tissue in which the tendon is housed and through which it moves.
A covering of synovial membrane surrounding certain tendons.