FFG doesn't seem to be big on explicit definitions, at least in D2E …
FFG has a reputation for not always being as explicit as they could be about their rules.
is there a definition for a "triggering condition"?
A "triggering condition" is defined as the condition after which the card is allowed to be played, based on the text the card contains. Generally speaking, it is not difficult to tell what the triggering condition of a card is, although sometimes the precise timing of a triggering condition can get murky.
1) "Play at the start of your turn…" The triggering condition is the start of your turn. The "start of your turn" is widely accepted as being up until you begin activating monsters, if I'm not very much mistaken. You may only play one copy of this card per turn, since your turn only starts once for each turn you have.
2) "Play when activating a monster…" The triggering condition is you activating a single monster (NOT a monster group, mind you.) You can only play one copy of this card per monster activation, but you may play several copies of the card per turn, each on a separate monster which you activate. Note in general that monster groups are not activated, only individual monsters. The rules say you must activate all monsters belonging to a group in sequence, but you don't actually activate the group as a whole.
3) "Play during an attack…" The triggering condition is that you are in the midst of making an attack against an enemy target. You may only play one copy on this card per attack, however, if a single monster is able to make two attacks in one turn, you may play a copy of this card on each attack, if you have two of them. Generally speaking, you may wait until after dice are rolled to use the card, unless it specifically says "play before dice are rolled" or "play when declaring an attack." The term "during" an attack is usually meant to refer to the step where surges and abilities are used, which is after dice are rolled.