Introducing, addressing, or discussing things not relevant to or concerned with the subject at hand. Sometimes hyphenated (always if used immediately before a noun). Make sure you don't go off subject during your lecture, or you'll just confuse your students. This is off-subject a bit, but what do you think about the recent economic trend in the Asian markets? My father always includes this off-subject remarks whenever he tells a story, and it just drags the whole thing out for what feels like an eternity.