A slang term used among musos for a distorted two-note diad played on the guitar, usually but not always containing the root and the fifth. In heavy metal it often includes the root and the third, and very occasionally the third and the fifth. People often call any fifth chord with any number of notes a power chord, and really there's no problem with this as it's just a slang term anyway, but if someone was to say: "just playing power chords" it would be understood that it was diads being asked for.