Precast is mainly a building word. Precast concrete has already been poured into a shape — a wall, a beam, a stair, a panel — in a factory or yard, left to go hard, and only then taken to the building site. Workers lift the pieces into place instead of pouring wet concrete into a mould on the site itself. People also use precast as a noun for one of those ready pieces, and as a verb: to precast a panel is to make it that way. Though actually, the idea is not only concrete; you can precast other materials in the same spirit, but in ordinary talk it almost always means concrete. It sounds technical, not fancy. Builders like it because the weather on site matters less, and the shapes can be made more carefully. The everyday opposite is concrete cast in place, poured where it will live. Do not hear “cast” here as the people in a film; this cast is the old making-sense: to pour something into a mould and let it take that shape.