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.

Description sourced from Wikipedia. Images sourced from Wikipedia.