Teeth per Inch
From DT Online
The coarseness of saw blades is measured as the number of Teeth per Inch - or TPI. This is measured between the bottoms, or gullets, of the teeth. Typical values for commonly used saws are as follows:
Note: TPI can also be an abbreviation for Threads per Inch
Alternatively blades can be measured in Points per Inch, which is similar, but measured between teeth tips and so is always one number greater. Circular Saw blades are measured by the total number of teeth around the circumference.