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.

Discussion:
Choosing the Right Weight: Text vs. Regular
Golden Section as a Method for Finding Regular and Bold Complements
Erik Spiekermann explains the German system of weights
Interpolation curve for multiple weights

Related Articles:
Thomas Phinney laments that "weights" doesn't mean what it used to