Genes that are homologs but found in different organisms. That is to say, a pair of orthologous genes might arise from a speciation event rather than a duplication. So most genes have equivalent, ortholog, counterparts in the genomes of other organisms.

A pair of paralogs that diverge in different species will then have the more complex relationship of an orthologous pair of paralogous pairs! For example, there are several globin genes that are paralogs, and these globin genes will have orthologs in other species.