Yep - wraps per inch.
Some people swear that you have to use a WPI tool, but that's nonsense. A regular ruler will work just fine. Just wrap the yarn around the ruler over and over again, with the wraps sitting snug against each other, then count how many strands are as wide as an inch.
Here's a website about measuring WPI yourself:
Here's the guide for yarn widths:
Laceweight - 18 wraps per inch
Fingering - 16 wraps per inch
Sportweight - 14 wraps per inch
Worsted - 12 wraps per inch
Bulky - 10 wraps per inch
Super Bulky - 8 wraps per inch