A girder is a long, strong beam, usually of iron or steel, that carries weight in a building or a bridge — the kind of member that holds up a floor, a roof, or the roadway. You see them as the big I-shaped or H-shaped pieces in a half-finished building or under a flyover. Builders and engineers use the word; in everyday talk people may just say “steel beam.” A wooden beam that does the same job is more often called a beam, not a girder. Though actually, older writing also used girder more loosely for any main supporting beam.