Extension (mm): The actual length of the stem from the center of the steering bore to the center of the handlebar clamp. We often refer to this as the stem length
Reach (mm): The effective length of the stem measured from the centerpoint of the steering bore to the center of the handlebar clamp without accounting for rise. On stems with built-in rise this measurement will be made along an imaginary line (as seen in the diagram).
Angle A: Measured from an imaginary line perpendicular to the steering axis to the center of the stem extension.
Angle B: Measured from the center of the actual steering axis to the center of the stem extension. This is what we usually refer to when referring to the stem angle.
A simple way to look at this difference is the following: Angle B = Angle A + 90 degrees. For example, if a manufacturer advertises a 7 degree angle using Angle A, we may describe this as 97 degree.
Rise (mm): The effective height gained as a function of the stem's angle and length. Measured vertically from the center of the handlebar clamp to an imaginary line that bisects the central meeting point of the extension and vertical portion of the stem.
Height (mm): The overall length of the vertical portion of the stem.