This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.
f focal length
So distance from front principal point to object (subject)
Sfar distance from front principal point to farthest point in focus
Sclose distance from front principal point to closest point in focus
Si distance from rear principal point to film (image) plane
N f-number or f-stop
Ne effective f-number (corrected for bellows factor)
c diameter of largest acceptable circle of confusion, or the diameter of the circle of confusion
h hyperfocal distance
Here we use the more technical term "object" for the thing being focused on. Informally it is equivalent to the subject. See the technical notes at the end for more information on object distances, more information on the meaning of f-number and limitations to be observed when applying these formulas to lenses in which the aperture does not appear the same size front and rear.