or raygun, although most self-respecting weapons designers and sf books and films refuse to use the term, inventing a whole load of technical jargon to explain their own word for the concept.
The idea of a directed energy weapon is that some form of energy-carrying beam (like a laser or maser) is used to transport destructive amounts of energy to the target. The energy must be able to be absorbed by the target or it is ineffective. Defences against directed energy weapons include reflecting or re-directing the energy away from the target, or spreading the energy throughout a large area like a heat sink.