What's the difference between a pipe and a tube?
More often than not, people guess it has something to do with the quality of the materials, but that's got nothing to do with it. The difference between a pipe and a tube is how they are measured, and ultimately what they are used for.
A pipe is a vessel - a tube is structural.
A pipe is measured ID - a tube is measured OD.
How they are measured... Pipes are measured ID or inside diameter because they are vessels. Tubes are measured OD or outside diameter because they are structural.
Pipes have a consistent ID regardless of wall thickness. In other words, a 1/2" high pressure pipe may need a 2" thick wall, but the ID will still only be 1/2" even tho the OD is 4.5".
Generally speaking, a tube will have a consistent OD and it's ID will change. Engineers see tubes and pipes with different eyes.
A tube is structural.
By having a consistent OD they can vary wall thickness, changing the ID, to increase strength. Because they are consistent OD, they have predictable characteristics.
Again, the difference is simple, it's how they are measured and what their intended uses are.