Combinatio nova, abbreviated comb. nov. (sometimes n. comb.), is Latin for "new combination". It is used in taxonomic biology literature when a new name is introduced based on a pre-existing name. The term should not to be confused with species nova, used for a previously unnamed species.
There are three situations:
When an earlier named species is assigned to a different genus, the new genus name is combined with of said species, e.g. when Calymmatobacterium granulomatis was renamed Klebsiella granulomatis, it was referred to as Klebsiella granulomatis comb. nov. to denote it was a new combination.