ERD, the Effective Rim Diameter is the diameter on which the ends of the spokes should ideally lie in the finished wheel. Some rim manufacturers publish ERD figures, but it is not always clear exactly how these relate to the geometry of a particular rim's cross-section.
Conventionally the spoke-ends in an ideal, fully tensioned wheel are expected to stop flush with the nipple heads, but this is not an undisputable norm to strive for. For one thing, it means that there are then only a couple of turns "left" in the spoke before it will bottom out, so the "overshoot" tolerance is minimal. Furthermore, for single-bottom rim types, it is unavoidable that protruding spoke ends increase the risk of inner tube punctures.
The length of the threaded part (and thus the maximum adjustable length) of a typical spoke is 9-10 mm/20-22 threads. When the spoke end is flush with the nipple head the maximum 8 mm/18 threads are engaged (= the entire threaded portion of the nipple). Using a corrected ERD definition of 2 mm/4 threads short of flush (2x2 mm diametrically) results in greater adjustment tolerance (+/-2 mm) without undue risk of thread-stripping in the worst "undershoot" case of only 4 mm/9 threads engaged in all.
P is the flange diameter, or more exactly, the diameter of the circle described by the spoke holes in the hub flange; sometimes (incorrectly?) called pitch. Since it is defined as the distance between centers of opposite holes it does NOT equal the outside diameter of the hub's flange. Left and right flange diameters are usually, but not always, the same.
RID is the rim's inner diameter. You can measure this directly across the rim (averaging several measurements) or, if the ERD is known, you can put a nipple of known length into a spoke hole in the rim, measure the part of the nipple that protrudes and do a little arithmetic: RID=ERD-2(NL-NPr).
OLD The Over Locknut Dimension is the outside distance from the locknut on one side to the other. Ideally the same as the distance between the frame's droputs. Note that axle length is inconsequential in this context.
L This is the distance from one side's locknut to the center plane of the nearest flange. This value may or may not be the same on both sides. L1=(OLD/2)-W1; L2=(OLD/2)-W2.
W The term Width refer the distance between (the centre planes of) the flanges. For the purpose of trigonometrical spoking analysis this parameter is often divided by the wheel's centreline into its bilateral sub-parts, here designated W1 and W2 respectively. W1+W2+L1+L2=OLD.
NL Nipple length, i.e. overall length of the nippel used.
NPr Nipple Protrusion, i.e. length of the part of the nipple that can be seen outside the rim.