What is thread pitch?
The fetched Omni thread pitch page defines thread pitch as the distance between identical points on two adjacent threads, such as crest-to-crest or root-to-root.
How to measure thread pitch
Omni notes that one practical way to measure pitch is to measure across multiple crests and divide by the number of intervals between them. This is why the distance mode below uses length divided by threads minus one.
Pitch and TPI
The source page also works as a threads-per-inch calculator. In inch units, pitch is just the reciprocal of TPI.