Tendinitis is the inflammation or irritation of a tendon, the thick fibrous cords that attach muscle to bone. It often results from repetitive motion or overuse, leading to pain, tenderness, and limited mobility in the affected area. Common sites include the elbow (tennis elbow), shoulder (rotator cuff tendinitis), and Achilles tendon in the ankle. Treatment typically involves rest, ice, physical therapy, and anti-inflammatory medications.