Maybe I can help. A safety is a legal shot attempted for defensive purposes. Intentional fouls (picking up the cue ball and handing it to your opponent, directly pocketing an opponent's ball in 8-Ball, directly pocketing the cheese in 9-Ball when it's hanging in the pocket and you can't see your object ball, etc.) are not considered safeties, but are defensive plays.
But people say "defense" out loud when playing a legal shot for defensive purposes, that's my peeve (admittedly slight). They usually do not say anything when they pick up the cue ball (a much larger peeve, suitable for its own thread) or intentionally hit the wrong ball; people just mark the defense because it's quite obvious.
APA players also say "defense" instead of "safe" in non-APA leagues and tournaments. (Still a very small pet-peeve.)
As I said, it's a small pet-peeve of mine and in no way detracts from my enjoyment of pool. Since the scorekeepers know what to do when I say "safe" it has yet to be an issue.