The longest bone in the human body is the femur, commonly known as the thigh bone. The femur extends from the hip joint to the knee joint and is not only the longest bone but also one of the strongest. It plays a crucial role in supporting the body's weight during standing, walking, and running. The femur is essential in mobility and stability, as it connects to various muscles, including the quadriceps and hamstrings, which aid in movement. Its length and structure enable it to withstand significant stress, making it vital for proper locomotion.