Metal tubing is designated by its outside diameter, pipe by its inside diameter. For example, a 4 inch pipe will have 4.5 inch OD. There are cases of tubing made to pipe size specs, which makes everything more complicated. Often times the terms are used interchangeably, especially for small sizes. Larger sizes are likely to be referred to as pipe.

Pipe Sizes - Clearing Confusion
The exact wall thickness of any one schedule changes with the pipe size. ... So, given all the possible tube and pipe sizes and all the various names used ...
www.nomma.org/support/Pipe Sizes/Pipe Sizes.html