LS,
Well perhaps you had better go tell the entire plumbing industry that the 3/4" copper and 1/2' copper water "pipe" they are using by the mile, everyday, is not what they or the industry says it is. Pipe is measured, approximately, on the ID and tubing is measured on the OD. Even if they are the exact same thing, they will be named differently if they are discussed as pipe rather than tubing. The words "tubing" or "pipe" must be used to select the category.
You can measure 3/4 copper pipe all day and you won't find any measurement that equals 3/4". It's the same product as 7/8" tubing. Same with 1/2" and 1" copper. If you want 3/4" tubing, you can't call it pipe because it's different than 3/4" copper pipe, 3/4 iron pipe, 3/4" conduit or 3/4" PEX. But there is one thing you can do and that is argue that they are all the same, if you want to.
If you want to discuss the OD as in refrigerant tubing, call it tubing. If you want to talk about copper pipe in homes, used for water, call it pipe. The rule even applies to iron pipe. Measure it all day and you won't find a 3/4" OD measurement on 3/4" pipe, it's closer to 1'' for brass and 1 1/16" for galvanized, but you can discuss iron in terms of the word "schedule". It has an approximate 3/4" ID adjusted by the "schedule", or wall thickness. Just as you can or must discuss copper in L, M or K. All the same OD, but with different wall thicknesses. Those terms say nothing specific themselves, but refer you back to a chart if you want to know the exact dimensions they are refering to. Copper pipe, used for sweating, and sometimes refered to as tubing, is not discussed in terms of "schedule" That is for threaded or **** welded pipe, generally of steel or stainless. Copper tubing is discussed in terms of OD and wall thickness in thousandths. So, you have wall thicknesses in thousandths, Schedules and Types, depending on the material and the function. Whew!
Technically, and in engineering circles, tubing is a fine word and applicable. Perfectly OK to say that copper pipe is really tubing and that it is really a different measuement than the industriy uses. Go ahead if you want to, it's OK .
The best engineeers understand how material measurements and standards are arrived at. They understand the properties of materials and the best ones for specific uses. They also understand what industry needs, uses and has settled on for practical use. They are able to converse in both of these worlds and are successful because of it. It's so much easier to simply suggest a material in response to a simple question, Like the OP asked, than to argue about arcane enginering criteria unrelated to getting air from one side of the shop to the other.
You might want to go and argue that there is no such thing as a 2X4 or a 4X6 to a carpenter. Maybe you're technically right, if only refering to the exact measurements of boards, but so what? How does that get the house built? I can see the argument now as you pull out a ruler and argue that the measurement is really 1 1/2" X 3 1/2" and NOT 2X4, at least in the one you happened to pick up and measure. The same word can be used to name something, that is used to describe something. So what?
I'd like to overhear the conversation next time you ask, at the paint store, for "white" paint. "What color white?", "white", "bright white", " white", "gloss white", " white", etc. Then you can give him the definition of white to clear it up.
Then you can argue that a 3/4 ton truck is not a 3/4 ton truck. Is that a name, a capacity or a weight? The next time your neighbor casually mentions that he may get a new 3/4 ton truck, stop him and tell him how the Society of Automotive Engineers describe those vehicles behind closed doors and that they should only be refered to in that way!
So, carry on and the next time you are in Home Depot, tell the guy you want 7/8" schedule 40 copper pipe. "Wait, while I get the manager", is more likely than "OK". Then, at your next engineers convention, be sure to tell them that homes all across the Country are being fraudulently built with the wrong size pipe that was measured approximately on the inside rather than the outside, and further, the contractors don't know the size of the pipe they are using!
If you get a new air conditioner, ask the technician what "schedule" the copper tubing is. "It's not measured in schedule, it's type L", is his likely answer. He'll respond in tubing OD and possibly wall thickness.
Industries become efficient by streamlining and standardizing. Workmen and suppliers gravitate toward fewer standard parts and dimentions. But engineering standards must be called out in some precise way that can be agreed upon across the country or around the world. Two different situations.
Pipe and tubing mean essentially the same thing in the dictionary. The same shape and function. But in the field they have different meanings. It's the easiest way to alert the listener that you are discussing a material configured and measured in a special way for it's category and purpose. Don't say pipe if you mean tubing. Don't say tubing if you mean pipe. Pipe is ID, tube is OD. To make this mistake at the plumbing supply will just lead to confusion and reveal that you are not aware of industry standards. But, of course, when the guy tries to help identify what you mean, be sure to start lecturing him about how engineers identify things at conventions. Remind him you know more about it than he does. Then they'll quickly understand what you mean.
Then thread up a bunch of iron pipe and be done with it. Just don't use PVC. Uh oh, PVC, a potential other topic. 
