Metallicity in its general sense is the amount of Metals (as per astrophysics, Lithium (Li) and all heavier elements) in an astronomical object as compared to the whole, i.e., metals plus non-metals, Hydrogen (H) and Helium (He). Metallicity is of interest for stars, Globular Clusters (GC), galaxies, Galaxy Clusters (CL), Molecular Clouds, etc. Since heavier elements are synthesized over time, and the universe was nearly all hydrogen at the Big Bang, metallicity expresses an object's age, history, or genesis. A galaxy with high metallicity has lived long enough to gain it and a star with high metallicity may have been formed from gas from previous stars.
In metallicity's strictest sense, it is expressed by the letter Z which is defined to be the mass ratio of metals to all elements, with X representing the mass ratio of hydrogen to all elements and Y similarly for helium, and X + Y + Z = 1.0. The Sun's Z value is still under study but is around .02.
Metallicity of other objects is often expressed differently, in what could be called Metal Abundance, which relates the ratio of the count of metal atoms to all atoms of the object to the same for the Sun. This is expressed in Bracket Notation:
[M/H] = log10(Nmetals/NH)body - log10(Nmetals/NH)Sun
where Nmetals is the number of metal atoms and NH is the number of hydrogen atoms. As such, "[M/H] = 0" means "same metal abundance as the Sun".
This is often approximated by measuring the abundance of a specific metal that can be measured in a practical manner, typically Iron (Fe). Consequently, the abundance specifically of iron (the ratio [Fe/H]) is often used as a proxy to to express the metallicity of stars, galaxies, etc. The metallicity Z can be approximated by multiplying [Fe/H] by a number in the .9 to 1.0 range.
Stars can be categorized are into three groups according to metallicity, known as Stellar Populations:
In gas, metallicity affects optical thickness: the higher the metallicity, the optically thinner.
The Abundances of other metals in stars, etc., are often stated relative to iron, e.g., [Si/Fe] or [O/Fe]. When [Fe/H] is also established, abundances of these elements relative to hydrogen are evident. Oxygen (O) is more common in higher mass stars and Carbon (C) in lower mass. A [C/O] of 1.0 is high.
Habitable Zone (HZ)
Main Sequence Fitting
Milky Way Chemical Evolution
SkyMapper Southern Survey (SMSS)
Stellar Age Determination
Stellar Parameter Determination