Metal (occasionally plastic) pipe or tubing through which wires are run. Available in sizes from 1/2" to 6". The size, type, and quantity of wires permitted in a specific conduit is dictated by the NEC. See pipe fill.
https://amftgs.com/steel-conduit-electrical-fittings-info/
https://amftgs.com/steel-conduit-electrical-fittings-info/