The amount of force taken on unit area. So a force of 300 on an area of 5 imposes a stress of 60, if uniformly applied. In many members the stress is non-uniform. For example, in a beam the stress on a vertical section varies from maxima at the upper and lower faces to nothing at the neutral layer.
In bridge design, the materials available have been tested, and their behaviour under stress is known. The limiting stresses under steady and varying loads are known and recorded. Where materials are subjected to fluctuating stress they may fail due to fatigue, at stress values well below that sustainable in steady conditions.