Be careful, there is more than one interpritation of "offset." The correct version is what was mentioned by other respondents here - the distance from the center of the wheel to the bolting surface.
But some people think that offset is the distance from the back of the wheel to the bolting surface. This is incorrect, but it is believed by a lot of people who should know better, including people who work in wheel shops.
I had my original Volvo wheels widened to 6" by a shop that does nothing else, and I nearly had a disaster because I asked them to make them with a certain offset, assuming they were going to measure from the middle. They were going to measure from the back, and it would of resulted in a completly different wheel. Luckily, I caught them before they welded my wheels. I have talked to other people at other wheel shops and they think offset is measured from the back too. Yet all wheel manufacturers measure from the middle. I think the problem is a lack of education, and the fact that it is easier to measure from the back rather than from a theoretical center plane.