Orthology Describes the evolutionary origin of a locus. Loci in two species are said to be orthologous when they have arisen from the same locus of their common ancestor.
For example, gene A in species 1 and 2 are orthologous. In contrast, gene B1, which has arisen by gene duplication in species 2, is paralogous to gene B in species 1.

See also the figure at NCBI.