In optics, a condenser may be defined as a converging system designed to concentrate light radiating from a source on an object, creating a wide, powerful light beam. There are two types of condensers: dioptric and catoptric, the former employing lenses, the latter mirrors.

In electricity, a condenser is an instrument for accumulating electrical charges. It basically consists of two conductors (often metal foils) called armatures, separated by an insulator (such as glass, ebonite or paraffin paper). Leyden jars are simple cylindrical condensers.