168. When a cold air mass and a warm air mass meet, and there is no horizontal motion of either air mass, it is called a (n) __________.

A stationary front is defined as the boundary between two air masses, typically cold and warm, when neither air mass exhibits significant horizontal movement. This aligns with the question's description of air masses meeting without horizontal motion, distinguishing it from cold, warm, or occluded fronts which all involve movement.