FontWeight

FontWeight is an option for Cell, StyleBox and StyleForm which specifies how heavy the characters in a font should be.

Typical settings are "Plain" and "Bold".

For some fonts and on some computer systems, additional settings are supported, such as "Thin", "Light", "Medium", "SemiBold", "Heavy", "Black" and "Fat".

See notes for FontFamily.

See The Mathematica Book: Section 2.8.9 and Section 2.10.10.