The femur is the longest bone in the adult human skeleton.
This strong, cylindrical bone runs through the thigh, connecting the hip joint to the knee. Its upper end includes the rounded femoral head, which fits into the pelvis to form the hip joint; its lower end helps form the knee.
The femur is also the body's largest bone by volume and one of its strongest. It transfers forces between the trunk and the lower leg during standing, walking, running, and jumping. Thick layers of compact bone help it withstand substantial loads.
A common mix-up is calling the thigh itself the femur: the thigh is the body region, while the femur is the bone inside it. In children, the femur grows from cartilage-based growth plates near its ends, which later close as skeletal maturity is reached.