This may seem to be a simplistic definition, but I'm going to argue that it effectively encapsulates both the overall characteristics of all 2.101 definitions of commercial items and the reasons for commercial item exceptions from public law compliance.
The first part is fairly straightforward - it's something that can be sold to the general buying public. This means it's available - or could be available - for sale without restriction to nongovernmental entities. You may say, "But we can't buy services off of a GSA Schedule without permission, and those are commercial items"!