Synovial sac which surrounds parts of one or more tendons. Examples: synovial tendon sheath of manual digit 2, radial bursa.
Comment:
Alternate def: A synovial sheath is a layer of a tendon sheath containing tendons in the hand and foot. They lie internal to the fibrous tendon sheaths. An example is the common synovial sheath for the flexor tendons. [Wikipedia:Synovial_sheath]