A material with electrical conductivity of less than a conductor
and more than an insulator
A pure semiconductor tends to be like an insulator with zero electrical conductivity, but once light, heat, or impurities are applied, electricity flows through, like a conductor.
A conductor refers to “a material through which electrical charge can flow”, like iron, copper, gold, silver, and aluminum. An insulator is “a material without electrical conductivity” like glass, plastic, rubber, and diamond.
A semiconductor allows electrical flow like a conductor when certain impurities are applied. What makes a semiconductor different from a conductor is that the conductivity level can be controlled in a semiconductor.