The Collaborative International Dictionary of English v.0.48:

Bursa \Bur"sa\, n.; pl. Burs[ae]. [L. See Burse.] (Anat.)
   Any sac or saclike cavity; especially, one of the synovial
   sacs, or small spaces, often lined with synovial membrane,
   interposed between tendons and bony prominences.
   [1913 Webster]