If you've ever had a teacher who was particularly stern and strict, you might have (quietly) remarked on his austerity. When you're talking about the character of a person, austerity means "sternness and severity."
Austerity's original meaning, taken from the Greek word austeros, was "bitter or harsh taste." While it's rarely used that way anymore, it's still a great description of the word's current meanings. As well as describing a personal quality of sternness, it also means "a simple, plain manner" and "an extreme frugality." So you could talk about the austerity of a very plainly decorated room or the austerity you're forced to live with ever since your parents stopped giving you a weekly allowance. If your lack of money had a taste, it might very well be "bitter or harsh."