Wikipedia
In molecular biology a pan-genome (or supra-genome) describes the full complement of genes in a clade (typically applied to species in bacteria and archaea), which can have large variation in gene content among closely related strains). It is the union of the gene sets of all the strains of a clade (e.g. species). The significance of the pan-genome arises in an evolutionary context, especially with relevance to metagenomics, but is also used in a broader genomics context.
The pan-genome includes the "core genome" containing genes present in all strains, a "dispensable genome" containing genes present in two or more strains, and finally "unique genes" specific to single strains. However, these distinctions are not completely objective, since they depend on which genomes are included in the analysis.