The relationship between Anger and Aggression.

[OnlyNatural]

What about when anger is kept inside boiling? Is that not anger then? Is that intense resentment or something, and it only becomes anger when it is let out?

It's think it's still anger, but like any emotion, anger can have different 'flavors' (resentment, like you said, and also moral outrage, humiliated fury, bitterness, scorn/contempt), depending on the eliciting factors and the situation.

And then there's the anger that, when kept inside, gets turned around on the self in the form of self-hatred and self-destructiveness, versus other-directed aggressiveness.