Indices : Terminology : Weight
The visual darkness or lightness of the strokes in a character: ultralight, light, book, medium, regular, semibold, bold, black, ultrablack, extrablack, etc.
Choosing the Right Weight: Text vs. Regular
Erik Spiekermann explains the German system of weights
Golden Section as a Method for Finding Regular and Bold Complements
Using Multiple Masters to Generate Weights in a Family - Insight?
Interpolation curve for multiple weights
Thomas Phinney laments that "weights" doesn't mean what it used to