(sometimes re-bar) - Actually deformed reinforcing bar. A length of steel, round, and with ridges to allow concrete to hold it in place. The actual diameter of the bar is larger than the 'nominal' size. The 'nominal' size is the smallest diameter of the bar, where the actual size is to the extents of the ribs or deformities. A #4 rebar, 4/8" 'nominal' size, is actually 9/16" outside diameter.
Usage: Rebar is used in concrete bridges to keep the concrete from breaking under the weight of cars and trucks.