An ankle is a joint in the Human body, connecting the leg to the foot.
In the Federation, prisoners are required to wear ankle monitors on their ankles, so they will be detected if they manage to escape. Tampering with it alerts Federation authorities immediately. (VOY: "Caretaker", "Non Sequitur")
In 2368, Captain Jean-Luc Picard broke his ankle after the turbolift he was on with Jay Gordon Graas, Patterson Supera and Marissa Flores came to a violent halt when the USS Enterprise-D struck a quantum filament. (TNG: "Disaster")