It's called ganglion or cynovial cyst that contains mucopolysaccharide-rich fluid within a fibrous capsule; usually attached to a tendon sheath in the hand, wrist, or foot, or connected with the underlying joint. The cyst can be surgically removed under local anaesthesia if it really bothers you.
Hope it helps.
Alla Cranham, MSc ABMT,
Registered Homotoxicologist
Mercury House
Northgate
Nottingham
NG7 7FN
Tel +44(0) 845 450 7316
Mob +44(0) 77361 47458
Fax +44(0) 115 9163109
http://www.in-vivo-health.co.uk