The diameter of the screw at the crests is called:

Minor diameter

It is the smallest diameter of an external or internal screw thread.

It is also known as the core or root diameter.

Pitch diameter

In a nut and bolt assembly, it is the diameter at which the ridges on the bolt are in complete touch with the ridges of the corresponding nut.

It is also called an effective diameter.

Pitch

It is the distance from a point on one thread to the corresponding point on the next thread.

Lead

It is the distance in which a screw thread advances axially in one rotation of the nut.

It is equal to pitch in case of single start threads; it is twice the pitch in the double start, thrice the pitch in the triple start, and so on.

Crest

It is the top surface of the thread.

Root

It is the bottom surface created by two adjacent flanks of the thread.

Depth of thread

It is the perpendicular distance between the crest and root.

Flank

It is the surface joining the crest and root.

Angle of thread

It is the angle included by the flanks of the thread.

Slope of thread

It is equal to one-half of the pitch of the thread.