Screw thread terminology

Pitch diameter-

The pitch diameter (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.

Major diameter-

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.

Minor diameter-

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.

Crest-

The crest is the height at which an external thread is raised or the depth at which an internal thread is indented.

Flanks-

The flanks of a thread are the straight sides that connect the crest and the root.

Root-

The root is the bottom of the groove between the two flanking surfaces of the thread whether internal or external.

Angle of a thread-

The angle of a thread is the angle between the flanks, measured in an axial plane section.

Pitch of a thread-

The pitch of a thread is the distance, measured parallel to its axis, between corresponding points on adjacent surfaces, in the same axial plane.