an obsolete name for a biological species or genus which has been replaced by a new name

often misspelled as "basinym"] A name at its introduction; changes in rank and/or position create new combinations of names, but do not alter the basionym. See Art. 33, para. 2

In botanical nomenclature, the basionym of a name is its name- or epithet-bringing synonym. In short, the synonym it is based on.