[Thor Joachim Haga]

The terms are used interchangeably (and I’m guilty of that sometimes too, like in my first post here), but Sigbjørn is right, they have different meanings. Atonal is mostly associated with the new Viennese school, I think – 12-tone, Schönberg, Berg, Webern, that kind of stuff. While dissonant is a more general term for discordant phrasings. So you can have dissonant scores that are not atonal, for example.