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, Paterson Supra, and Marissa Flores came to a violent halt when the USS Enterprise-D struck a quantum filament. (TNG: "Disaster")
In 2369, while suffering from an aphasia virus, Chief Miles O'Brien babbled to Major Kira Nerys, "ankle, try sound, reset gleaming." (DS9: "Babel")
A verse in a Klingon drinking song contains the line "And the blood was ankle deep." (DS9: "The Way of the Warrior"; VOY: "Barge of the Dead", "Prophecy")
In 2373, B'Elanna Torres insisted on continuing a hoverball championship despite the fact that she had a broken ankle. (VOY: "Remember")