A door lintel is a horizontal architectural support placed along the top of a door opening to keep the material above it from crashing down. Lintels are a key part of post-and-lintel construction, a simple form of architecture seen in many ancient buildings and structures.
The posts, in the case of a door, are the vertical sides of the door opening. Door lintels are not a pure example of post-and-lintel construction, because door lintels are part of the wall in which the door is placed. These lintels are subject to the forces placed on the entire wall.
In most modern construction, posts and lintels act as a single unit, bearing loads as one. A more quintessential example of door-and-lintel construction is seen in the form of the structures at Stonehenge.