In architecture, a lintel is the beam or other support at the top of a door or window. Most lintels are decorative as well as providing structural support.
A lintel can be purely ornamental, just an attractive horizontal flourish at the top of a door, or it can be a wood or metal support piece that also looks nice. Your fireplace might be topped by a stone lintel, and you might see a much older lintel in ancient Roman ruins, at the top of two massive pillars. In Old French, lintel means "threshold," from the Latin word limitaris, "bordering."