Say a couple has a good relationship. One day, Partner A has a bad day at work. A comes home and screams at Partner B: "you're dumb! you're slow! why isn't dinner ready? you're a dolphin!"

Partner A was just venting frustration after having a terrible day. Is that abuse? YES! Unequivocally, yes.

There is no excuse for insulting, yelling, or otherwise treating other human beings as lesser than. It's abuse, always, even one time. And frankly if it happens once, it's very unlikely it isn't a repeated pattern of behavior.