Could someone help me think of the term which describes this economic principle?

Basically, it is that within any economic situation, there are winners and losers. For example, when a Wal-Mart opens everyone wins with lower prices. However, for the unionized employee of a competitor whose job is eliminated, there are losers.

I’m not trying to turn this in to a Wal-Mart debate, that is just the only example which came to mind. I know there is a term or economic principle which applies to this theory, I just cannot remember what it is.

Can anyone help?