Technically, a dormer is a window in a raised section of the roof, but the term also refers to the entire raised section. There are two basic styles of dormers: a small raised section with a gabled roof, (they're often called a "dog house" or "pigeon coop" dormers),and larger "shed" dormers, which generally have flat (as opposed to gable) roofs to provide increased interior headroom.
A dormer is a window housed in a gable or similar structure affixed to the sloping part of a roof, providing natural light and ventilation to the rooms beneath the roof. Since such attic or garret rooms have traditionally been used for sleeping, the dormer gets its name from the French verb dormer: to sleep.
Roofed projection built out from a sloping roof into which a window is set to provide additional interior light and ventilation. The word dormer comes from the word dormitory, which was normally in an attic area.
A dormer is a structural element of a building that protrudes from the plane of a sloping roof surface. Dormers are used, either in original construction or as later additions, to create usable space in the roof of a building by adding headroom and usually also by enabling addition of windows.