The squeezing of material by external forces . Compression is always present somewhere in every bridge. When this kind of stress is present, tension and shear also occur. Columns and footings carry compressive loads.
In beams the upper part of the member is under compression. Piers carry compressive loads over their cross-sections. In girders the struts are in compression. Most materials are strong in compression, but a strut may fail by buckling , the member curving out of its original alignment. Demolition sites often provide examples of compression failure, though you should seek permission and tread warily, at such sites.