Rebar (short for reinforcing bar), collectively known as reinforcing steel and reinforcement steel, is a steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and hold the concrete in tension. Rebar's surface is often patterned to form a better bond with the concrete.
Any material with sufficient tensile strength could potentially be used to reinforce concrete. The most common are steel, glass fiber, basalt fiber, and fibre-reinforced plastic. Steel and concrete have similar coefficients of thermal expansion, so a concrete structural member reinforced with steel will experience minimal stress as the temperature changes.