What is a Q angle in the human body?

Answer 1

The Q-angle is the angle of the femur as it enters the hip socket.

http://moon.ouhsc.edu/dthompso/NAMICS/qangle.htm

The Q angle which is formed by a line drawn from the anterosuperior iliac spine through the center of the patella and a line drawn from the center of the patella to the center of the tibial tubercle, should be measured next. In women, the Q angle should be less than 22 degrees with the knee in extension and less than 9 degrees with the knee in 90 degrees of flexion. In men, the Q angle should be less than 18 degrees with the knee in extension and less than 8 degrees with the knee in 90 degrees of flexion.

Answer 2

As an addendum to Mercury's answer, "Q angle" stands for quadriceps angle. It influences the variability of the lower extremity and knee balance.