IPS simply means Iron Pipe Size. Piping is a general term used to describe any hollow, cylindrical carrier of liquids. Many piping materials are used to produce both pipe and tubing. Piping and tubing made of the same materials generally use different kinds of fittings.
All piping made to the OD (Outside Diameter) of wrought iron pipe is referred to as being Iron Pipe Size, or IPS. The OD of most pipes is the controlling dimension in order to match with fittings. When the term OD or (Outside Diameter) is used, the dimension being referred to is always the actual diameter unless proceeded by the phrase “nominal”.
Nominal size is a term used for the comparative sizes of piping and tubing rather than the actual dimensions. Like 3/4″ or 1″. Nominal sizes of piping in sprinkler systems are not the actual sizes and are given in inches and fractions of an inch. For a given NPS (Nominal Pipe Size), the OD stays fixed and the wall thickness increases with schedule.
For example, you go to the home improvement store to buy some sprinkler pipe and you find Schedule 40 and Class 200 sprinkler pipe available. Both pipes will have the same OD(Outside Diameter), but the Schedule 40 pipe will have a thicker inner wall than the Class 200. Schedule 80 pipe will be thicker than Schedule 40 pipe but will also have the same OD. The gray 1/2″ riser nipples you buy for sprinkler risers in flower areas are often Schedule 80 nipples.
So even though IPS is somewhat of a dinosaur term, it is still used when referring to the pipe size of the PVC pipe you are buying at the home improvement store. What makes it even nicer is that the Schedule 40 fittings you buy will fit all of these pipes, because all that is changing is the pipe thickness and not the OD (Outside Diameter) of the pipe.