An Etymological Dictionary of Astronomy and Astrophysics

The fractional amount (by mass) of a given → chemical element or → nuclide in a given → chemical composition.

In chemical composition studies of astrophysical objects
the mass fractions of → hydrogen, → helium, and all the remaining chemical elements are usually denoted by the parameter X, Y, and Z, respectively. Their sum is defined as X + Y + Z = 1. The parameter Z is usually referred to as → heavy elements or → metals.

See also: → mass; → fraction.