A buttress is an architectural structure built against or projecting from a wall which serves to support or reinforce the wall. Buttresses are fairly common on more ancient buildings, as a means of providing support to act against the lateral (sideways) forces arising out of the roof structures that lack adequate bracing.
The term counterfort can be synonymous with buttress, and is often used when referring to dams, retaining walls and other structures holding back earth.
In addition to flying and ordinary buttresses, brick and masonry buttresses that support wall corners can be classified according to their ground plan. A clasping or clamped buttress has an L shaped ground plan surrounding the corner, an angled buttress has two buttresses meeting at the corner, a setback buttress is similar to an angled buttress but the buttresses are set back from the corner, and a diagonal (or 'French') buttress is at 135° to the walls (45° off of where a regular buttress would be).
The gallery below shows top-down views of various types of buttress (dark grey) supporting the corner wall of a structure (light grey).