According to Oxforddictionaries.com, a louvre is each of a set of angled slats or flat strips fixed or hung at regular intervals in a door, shutter, or screen to allow air or light to pass through. Thus, a horizontal louvre is the part of a portable air conditioner that, as a set of horizontal angled slats or flat strips, can be adjusted by angling the slats or strips in an upward or downward direction in order to determine the upward or downward direction of air flow from the air conditioner into the desired space.