Tennis elbow, also known as lateral epicondylitis, is an enthesopathy (attachment point disease) of the origin of the extensor carpi radialis brevis on the lateral epicondyle.  It causes pain and tenderness over the bony part of the lateral epicondyle. Symptoms range from mild tenderness to severe, persistent pain. The pain may also extend into the back of the forearm. It usually has a gradual onset, but it can seem sudden and be misinterpreted as an injury.

Tennis elbow is often idiopathic. Its cause and pathogenesis are unknown. It likely involves tendinosis, a degeneration of the local tendon.

It is thought this condition is caused by excessive use of the muscles of the back of the forearm, but this is not supported by evidence. It may be associated with work or sports, classically racquet sports (including paddle sports), but most people with the condition are not exposed to these activities. The diagnosis is based on the symptoms and examination. Medical imaging is not very useful.

Untreated enthesopathy usually resolves in 1–2 years. Treating the symptoms and pain involves medications such as NSAIDS or acetaminophen, a wrist brace, or a strap over the upper forearm. The role of corticosteroid injections as a form of treatment is still debated. Recent studies suggests that corticosteroid injections may delay symptom resolution.

Patients typically feel pain or burning around the outer part of the elbow (lateral epicondyle of the humerus), which can move down the forearm and sometimes up to the upper arm. The pain is worsened by activities that involve wrist extension, such as gripping objects. Pain intensity varies from mild to severe and can be intermittent or constant, significantly impacting daily life. Patients also commonly report grip weakness and difficulty lifting.

The term "tennis elbow" is widely used (although informal), but the condition affects non-tennis players. More recently, with the explosive growth of pickleball, the term "pickleball elbow" is frequently used. Historically, the medical term "lateral epicondylitis" was most commonly used for the condition, but "itis" implies inflammation and the condition is not inflammatory. It is also referred to as enthesopathy of the extensor carpi radialis origin.

Since histological findings reveal noninflammatory tissue, the terms "lateral elbow tendinopathy" and "tendinosis" are suggested. In 2019, a group of international experts suggested that "lateral elbow tendinopathy" was the most appropriate terminology. But a disease of an attachment point (or enthesia) is most accurately referred to as an "enthesopathy."

The exact cause of lateral epicondylitis remains unclear. However, it is often linked to repetitive microtrauma resulting from excessive gripping, wrist extension, radial deviation, and/or forearm supination.