Tennis elbow AKA lateral epicondylitis is a painful inflammation of the tendons at the elbow caused by overuse of the forearm muscles, and resulting in restricted movement. It is similar to golfers elbow, the only difference being the location of the tendons affected.
Tennis elbow is a common symptom of RSI conditions, caused by repetitive, one-sided movements. For example, those carried out in a job such as carpentry, or a leisure activity such as gardening. Only about 5% of people diagnosed with tennis elbow have actually sustained the injury playing raquet sports.
Because the tendons affected are different, it is possible for someone to have both tennis elbow and golfers elbow at the same time. And not necessarily ever to have played tennis or golf!