I never really understood why they had to differentiate between "Legendary" and "Mythical". I think Mythical is supposed to refer to event-only Pokemon, but that seems to be a pointless distinction. I mean, Deoxys used to be an event-only Pokemon, but then in ORAS you can just catch one during the Delta Episode. So is Deoxys no longer a Mythical? Are they a Legendary now because you can catch one normally (but only in one game)?
Anyway, it kinda surprised me that the terms "Signature Move/Ability" aren't official terms because I could have sworn that I had seen official media using it. For example, I thought that I had seen a promotional piece about some Legendary Pokemon claiming they had a Signature Move, but I must have just misremembered their phrasing. Maybe they said something like "special move" or "move unique to [POKEMON]." Or maybe I just made the whole thing up altogether, who even knows?
"The human sacrificed himself, to save the Pokemon. I pitted them against each other, but not until they set aside their differences did I see the true power they all share deep inside. I see now that the circumstances of one's birth are irrelevant; it is what you do with the gift of life that determines who you are." -Mewtwo