(note: this page is far from complete, so be sure to check back later on)
I have found that the best results come from using a definition of ERD that is based on the spoke ends being flush with the bottom of the screwdriver slot on the top of a standard 12mm nipple.
(more info to come on how to measure ERD in this manner...)
Ignore angled or curved profiles and measure squarely across the widest point of the rim's cross section. A caliper will give the best results.
Note that many rim manufacturers advertise a width of their rims, and these stated values are commonly inexact. Sometimes, as is common with tires, manufacturers will intentionally state a wider than actual width in order to help have "the lightest 28mm rim" or some such nonsense. Needless to say, real-world accurate measurements are greatly preferred.
Measure across the narrowest point of the hooked edges of the rim.
Measure over the vertical cross-section of the rim.
Brake Wall Height
On rims that can be used with rim brakes, measure the height of the flat braking face, not including any curved portions at the edges. This measurement will probably be difficult to nail down to a high accuracy due to minor variations in machining and the lack of firm edges to measure against. A precision of about a half millimeter is acceptable.
The over-lock-nut dimension (also sometimes abbreviated as OLD) is the same as the inner spacing of the dropouts when the wheel is installed. This will usually be one of the common values of 100mm, 120mm, 126mm, 130mm, 135mm, etc.
In the context of newer axle systems such as thru-axles, the meaning of this number can get a little confusing. If there is a standard measurement used in industry to refer to the width outside of the hub and inside of the fork/frame, it is probably the best number to use.