Utility

Utility is how many "happiness points" that you get for making some decision.

The basis of any economic analysis of making decisions is comparing the utility and costs of the available options. The best economic decision is one that maximizes the utility gained from making that decision.

You can spend the $20 you have on tickets to a performance by some unknown Indie band or you can go to the movie theater and watch explosions in 3D. Both cost the same amount of money, but you might prefer one over the other, so you should make the decision based on which one maximizes your utility.