Merriam-Webster Collegiate Dictionary - louver

 or louvre  noun  Etymology: Middle English lover, from Anglo-French  Date: 14th century  1. a roof lantern or turret often with slatted apertures for escape of smoke or admission of light in a medieval building  2.  a. an opening provided with one or more slanted fixed or movable fins to allow flow of air but to exclude rain or sun or to provide privacy  b. a finned or vaned device for controlling a flow of air or the radiation of light  c. a fin or shutter of a ~  • ~ed also louvred adjective