This is the diameter of an imaginary cylinder parallel with the crests of the thread; in other words it is the distance from crest to crest for an external thread, or root to root for an internal thread.
Found on
http://www.boltscience.com/pages/glossary.htm
On a straight thread, the diameter of the coaxial cylinder which would pass through the crests of an external thread or the roots of an internal thread.
Found on
http://www.encyclo.co.uk/local/21038
Diameter of an imaginary cylinder that would bound the crests of an external straight thread or the roots of an internal straight thread
Found on
http://www.encyclo.co.uk/local/21059
No exact match found.
