"Horizontal movement" refers to the action or motion of something moving side to side or parallel to the ground in a straight line, rather than up or down. Full definition
You control the fan power, and therefore the height of the foam ball, with your thoughts; you control the ball's horizontal movement through the course with a knob. (scientificamerican.com)
Hit both the vertical and horizontal movement patterns of your back with this one single exercise. (fitstep.com)
Finally we have the atmosphere where again convection and horizontal movements allow the energy to be shifted around. (realclimate.org)