Mass fraction indicates the quantity of a particular constituent (or grouping of constituents) of a substance, as a fraction of the total mass. The term is used in chemistry as well as astronomy.
Mass fraction has a particular use in astronomy for the description of stars and clouds, with these common indicators:
Z is thus the metallicity by mass.
The mass fractions present after Big Bang nucleosynthesis are often listed with a P for primordial suffix: Yp, etc. The values are a topic of research but Xp and Yp are presumed both by theory and by extrapolation of observations to be roughly .75 and .25, with Zp being negligibly small. One means of determining that part of Y due to additions since the Big Bang is to survey for the ratio between changes to Y and Z over time (ΔY/ΔZ) and use that ratio to estimate how much current helium was synthesized post-Big-Bang.