abat-vent

Century Dictionary and Cyclopedia

1. n. A vertical series of sloping roofs or broad slats, inclined outward and downward, forming the filling of a belfry-light, and designed to admit ventilation to the timber frame while protecting the interior from rain and wind, and to direct downward the sound of the bells.
2. n. A sloping roof, as that of a penthouse: so named because the slope neutralizes the force of the wind.
3. n. Any contrivance designed to act as a shelter or protection from the wind. Specifically, a revolving metallic cap carrying a vane, attached to the top of a chimney to keep the wind from blowing directly down its throat.

Wiktionary

1. n. A sloping roof.
2. n. A metal chimney cap.