I've actually seen DLCs sold in stores. They were usually bundled and called "expansion(s)"...

To avoid confusion - this is NOT a snide remark. It seemed strange and hilarious to me as well, but it makes sense...

Usually when that is done, it is multiple DLCs bundled together so that the amount of content as a whole could be considered an expansion. Really the difference between the two is the amount of content involved. A DLC would be a map pack, or a new adventure that lasts a couple hours, or some new items while an expansion would have new maps, items anda campaign of significant length.