Definitions for "Orthologous"
A gene present in multiple species that descended from a unique ancestral gene, when the relative divergence in different species is comparable to the overall difference between the species. Such genes provide useful tools for constructing phylogenetic trees (for examples, see http://www.ncbi.nlm.nih.gov/COG/).
Homologous as the result of speciation but not gene duplication. Orthologous genes are derived from the same copy of a gene in the most recent common ancestor. Compare Paralogous.
Homologous sequences in different species that arose from a common ancestral gene during speciation.