Peroneal tendonitis is an inflammation or irritation of the peroneal tendons on the outside of the leg.
The peroneal muscles are two main muscles on the outside of the leg (lateral side) that insert into their tendons just about the ankle joint. The tendons pass behind the outside ankle bone (lateral malleolus) an then go for a short way along the outside of the foot before one inserts into the base of the fifth metatarsal and the other one goes around the cuboid bone to insert into the bas of the first metatarsal.
If these tendons become irritated or there is an overuse type injury, then you have a peroneal tendonitis. The typical symptoms for this is initially a mild ache either just above or below the lateral malleolus that can get gradually worse with exercise. In some cases there may be a bit of swelling.