Light is a wave: a disturbance spreading through an invisible medium, the ether, as sound spreads through air. It travels more slowly in glass and water than in air, which is why its path bends as it enters them.
Light is a wave of electric and magnetic forces, traveling through space at the speed my equations give for such waves, about 310,000 kilometers a second. Radiant heat, and any other radiation like it, must be such a wave too.