The frequency of an allele in a population is the number of copies of the allele present
throughout the population divided by the total number of alleles present for that factor.

More precisely, say that our a population contains $p$ individuals whose somatic cells
are $n$-ploid, meaning that chromosomes are divided into homologous
groups containing $n$ chromosomes each (humans and most animals are diploid, with $n = 2$).
If an allele is present in the population in $a$ copies, then its allele frequency is given
by $\frac{a}{n \times p}$.