… you realize and act upon the understanding that people make decisions based on their marginal costs, and their marginal benefits.

That’s the definition I was given when I was an undergraduate, and I still like it today. Since there is no official economist-certification board I think that’s as good as anything I have heard.

Super-economist, noted textbook author and Harvard professor Greg Mankiw has one of the best blogs anywhere. I think he understands this principle too. This is shamelessly lifted from his blog: