threads

The pitch diameter (often called the effective diameter) of a parallel thread is the diameter of the imaginary co-axial cylinder which intersects the surface of the thread in such a manner that the intercept on a generator of the cylinder, between the points where it meets the opposite flanks of a thread groove, is equal to half the nominal pitch of the thread.
The major diameter of a thread is the diameter of the imaginary co-axial cylinder that just touches the crest of an external thread or the root of an internal thread.
The minor diameter is the diameter of an imaginary cylinder that just touches the roots of an external thread and (or) the crests of an internal thread.
The crest of a thread is the prominent part of a thread, whether internal or external.
The root is the bottom of the groove between the two flanking surfaces of the thread whether internal or external.
The flanks of a thread are the straight sides that connect the crest and the root.
The angle of a thread is the angle between the flanks, measured in an axial plane section.
The pitch of a thread is the distance, measured parallel to its axis, between corresponding points on adjacent surfaces, in the same axial plane.