Final answer:
The term describing the perception of head orientation when the body is stationary is 'static equilibrium,' detected by the vestibular system.
Step-by-step explanation:
The term that refers to the perception of the orientation of the head when the body is stationary is static equilibrium. The vestibular system is responsible for detecting both linear acceleration (such as gravity) and angular acceleration and deceleration, which help us sense our head's orientation and balance. Static equilibrium involves a body in an inertial frame of reference at rest, where no rotational or translational motion occurs. This is distinguished from dynamic equilibrium, where the body is moving at constant velocity and angular velocity but with no acceleration.