(1) The typical (or the most common) form, appearance or strain existing in the wild.
(2) The normal, non-mutated version of a gene common in nature.
(3) The allele at each locus required to produce the wild-type phenotype.
The wildtype is the most common form or phenotype in nature or in a natural breeding population.
In genetics, the wild-type organisms serve as the original parent strain before a deliberate mutation is introduced (for research) so that geneticists can use them as reference to compare the naturally occurring genotypes and phenotypes of a given species against those of the deliberately mutated counterparts.
Variant: wild type (noun), wild-type (adjective)