One of the two taxa on either side of a split in a cladogram. In practice: the taxon to which another taxon is more closely related than any other taxon. Usually refers to sister species, i.e., taxa at the species level.
Does someone has a scan of Nartshuk E.P. 2003. Key to families of Diptera (Insecta) of the fauna of Russian and adjacent countries. Proceedings of the Zoological Institute Vol. 294: 1-252 for me?