Two weeks ago at small group, one of the fellows (who happens to be a psychiatrist) in the group shared this definition of anger: "Anger is being upset that you didn't get your way." I thought that was very insightful and definitely worth pondering.
I also think that definition could be expanded. Being irritable or frustrated is also being upset that you didn't get your way. As a society, we make distinctions between the three--anger, frustration, and irritability--as if to diminish or rationalize our right to feel that way (depending on the situation and who we're talking to). This definition really made me think about how justified--or rather that I'm actually unjustified in feeling these three emotions at times.