Think of it this way. Cheese is about taking advantage of a deficiency in the game's AI to gain an advantage. Give it the reality check - would it be realistic in real life? If the answer is yes, then it's not cheese. If the answer is no, then it is.
Using traps to kill Firkraag is not cheese if you set the traps at the end of the cavern and lured him there so he would set them off. It IS cheese if you set it in front of him by taking advantage of the fact that as a non-hostile he will do nothing despite the fact that you ring him with traps, sleeping for 3 days to make sure you lay enough. The former is a realistic scenario - the latter isn't. Same goes for casting Cloudkill or another AOE spell at beyond visual range and waiting until he chokes himself to death - you are taking advantage of a game limitation which prevents Firkraag from attacking you because you have not directly attacked him. Casting Cloudkill normally and killing him (and maybe you as well) is not generally considered cheese.
Here's a tougher one - casting cloudkill into a room full of monsters and shutting the door, waiting for them to choke, then opening the door again, casting, and shutting again, repeating till they've all been Baygoned. The door is NOT locked, but of course the AI doesn't allow monsters to open doors. Now you can try and justify this by saying that your characters are HOLDING the door shut, but it has the distinct smell of cheddar.
Hope that clears the definition up. Note that I didn't say anything about cheese being good or bad - it's neither.
[ 01-30-2002: Message edited by: Memnoch ]