I have a lump on the joint between my middle finger and hand. What is it?

Finger Joint Lump. This requires an exam to determine if it is on the top (wrist side) or palm side of the hand. Fixation or movement with hand open/close maneuver is also important. If it is on the top it is more likely related to an injury or poorly healed fracture; however there are other possibilities. On the palm, it is more likely a ganglion but could be early appearance of a nodule associated with dupuytren's.
Ganglion? Without seeing it is difficult to say for sure. If you have a nodule on the palm side of your finger at the base where it meets your hand it is most likely to be a ganglion. These can sometimes go away on their own but due to location and continued irritation caused by grasping it may require excision. This is an outpatient procedure usually done under local anesthesia. Good luck.