The femur is the thigh bone: the long, strong bone that runs from your hip to your knee. It is the largest bone in the human body, and it takes a great deal of force to break. Doctors, anatomy teachers, and accident reports use femur; everyday English is happy with “thigh bone.” Though actually, the same name is used for the matching upper-leg bone in other animals, so a bird has a femur too. You will see it labelled on diagrams. It is a school-and-hospital word, not a word you need at the dinner table — unless someone has had a fall.