cf. or confer

cf. or "confer" from Latin means "comparison" and is used exactly that way in the botanical context. If it is not possible or uncertain to identify the species of a plant, this abbreviation is placed between the genus and species names in order to at least express the next best suspicion. Of course, it remains unclear whether the plant in question is actually this species, especially since another unknown or undescribed species may also be present.
The species mentioned is therefore comparable in appearance and a reference to the plant in question in the broader sense is assumed.