What is the definition or description of: ankle sprain?

Ligamentous injury: Tensile injury to ligaments stabilizing the tibioitalar joint usually caused by the articulation being carried through a greater than normal range of motion.

See below: It is when excess stress or trauma forces the ligaments and tendons around the ankle to be stretched past their normal length causing pain and swelling.

Ankle Sprain: Ankle sprains are very common injuries. There's a good chance that while playing as a child or stepping on an uneven surface as an adult you sprained your ankle--some 25,000 people do it every day.​ http://www.aofas.org/footcaremd/how-to/foot-injury/pages/how%20to%20care%20for%20a%20sprained%20ankle.aspx