The major thread diameter is the distance from crest to crest through the central axis of a screw. The minor thread diameter is the distance from trough to trough through the central axis of a screw.
Standard screw thread callouts include other parameters, such as pitch diameter, pitch and several properties describing the shapes of the threads themselves. The pitch diameter is defined as the diameter halfway between the minor diameter of the thread and the diameter of a theoretical sharp created by the thread crests. Thread pitch is the distance from one thread to the next, or the axial length of one full thread rotation.