RPI, a three letter abbreviation (TLA) for Ridges-per-Inch, a crude but accurate measurement of the excellence of a solution. Despite its name, and the fact that it is stated as a numeral, the abbreviation can be used for just about anything, as it has no direct correspondence with ridges or inches.

According to Bane, this is the original definition of Ridges per Inch:

"Basically, the attractiveness of a thrashing oily metal remote-controlled hentai tentacle dildo hose (I'm sure you'll have no problems visualising it) is primarily governed by the ridge count. However, though RPI became the term of choice, there are plenty of other aspects to evil penetrative devices that can affect their overall desirability. Tell them, Amy!

Huh? Oh no, not Rose...

"Sure! Therefore, a specific device could have few ridges, but compensation in the areas of, say, electrical contact points or square-wave vibratory mechanisms could bump its overall RPI right past a really ridgy but otherwise conventional torture toy.
Back to you, oh blushing hedgehog!"