phenotypic variance
Phenotypic variance is a single number that captures how spread out a trait is across a population — how much individuals differ from the average and from each other. If everyone were the same height, the variance would be zero; the wider the range of heights, the larger the variance. It is the raw total of observable variation that all of quantitative genetics sets out to explain.
The central move in the field is to split this total into its sources. The most basic partition is genetic plus environmental: some of the spread comes from people carrying different alleles, and some from people living in different conditions. The genetic share can be split further into additive, dominance, and interaction parts, and a term can be added for gene-environment interaction where the two refuse to separate cleanly.
This bookkeeping is what makes heritability meaningful, since heritability is literally a slice of phenotypic variance expressed as a fraction. The honest caveat is that the partition is specific to the population and environment measured: a trait's phenotypic variance, and how it divides up, can look quite different in another group, in another place, or under another set of conditions.
If a wheat variety shows a large spread in yield, a geneticist plants genetically identical lines across the same field to see how much spread remains from soil and weather alone; subtracting that environmental variance from the total reveals the genetic share.
Total variation splits into genetic and environmental shares.
In symbols, the basic split is written Vp = Vg + Ve: phenotypic variance equals genetic variance plus environmental variance, with an interaction term added when needed.