I discovered while shopping at Home Depot that they had tubing that specified the size both ways. I thought that was odd at the time, but thanks to John's explanation, now I know why. One box was labeled 1/2 inch tubing, and it was 1/2 inch ID, 5/8ths OD. Right next to it was another box labeled 1/2 inch tubing that was 3/8ths ID, 1/2 inch OD, The latter was called "refrigerant" tubing, as John said.
You can decide pretty easily what size you want regardless of what the box says when you're standing in the ailse at Home Depot. I do wish that retail vendors of manufactured chillers would specify whether they are talking about ID or OD in their product listings. Maybe some do. MoreBeer does not. I bought a 50 foot B3 Superchiller
that they advertise as a 1/2 inch chiller, but fail to mention whether that is ID or OD. They are being truthful. It is 1/2 inch, but it is the refrigerant specification, so that is 1/2 inch OD, not ID. I had also made myself a 25 foot chiller (now my pre-chiller) from I/2 inch tubing. But mine is 5/8ths OD.
I doubt it really matters that much, but because copper tubing is labeled differently depending on whether it is refrigerant tubing or not, people who sell products made from copper tubing really should indicate whether they are referring to ID or OD.