A lifestyle free of drugs, alcohol and casual sex. It has nothing to do with abstaining from *all* drugs (such as prescribed medicines or even caffeine - in fact the original straight edge dudes from Wash DC used to get hyped up on Coca-Cola as a sort of joke - "whee we can get crazy just on regular soda!" sorta thing). It also has nothing to do with vegetarianism - although many people that are into straight edge are big time vegetarians (including myself). Any variation or expansion of/on the "big 3" (drugs, alcohol, casual sex) is not straight edge in the classic sense.
WrongKid#1 "I don't take aspirin because I'm straight edge".
WrongKid #2 "If you eat meat you can't claim the edge".
KidThatKnowsWhatsUp - "Straight edge is about abstaining from drugs, alcohol and casual sex, period. No more, no less. This desire to keep my mind and body sharp has also opened my eyes up to the virtues of vegetarianism/veganism, but since I'm not a dumbass and know a little bit of punk rock history I'm aware these additional things simply compliment straight edge, but are NOT straight edge per se".