A header is a beam that spans the opening for a doorway, window, skylight, or stairway.
Though the word “header” dates back to 15th-century England as the name given to an executioner, its earliest use as a building term occurred a couple of hundred years later. It did—and still does—refer to part of a stone or brick wall. It is a brick or stone that bridges an opening. The header is placed with its end, or head, facing the wall, a practice that fortifies a wall’s construction.
Today, that term has been adopted to the parts used in wood-frame construction.
Now, the word header refers to a beam-like support in wood-frame construction.
The header spans an opening for a window or door, or it is a cut-out in the roof or floor for a skylight, chimney, or staircase.
Typically at least twice the size of surrounding framing members, a header is often built up from two studs, joists, or rafters. It runs between full-length supports and, along its length, carries the loads of the shorter studs, joists, or rafters that terminate at the opening.