AKA wastebin taxon, dustbin taxon, rag bag taxon, trash can taxon, or catch-all taxon.
'Wastebasket taxon' is an informal but common term for the practice of using an official taxon as an easy place to dump unclassified species.
As a clear example of this, most snakes are placed in the family Colubridae not because they belong in a family together, but because at one point it was recognized that the taxonomic tree of snakes was so messy that it would be most wise to ignore the mess and hope that future generations would sort things out. Other classic examples include the order Insectivora, although it has since been dissolved, and the superclass Agnatha, which is in the process of being resolved.
You may also find less formal wastebasket taxons, as in the infamous archaic H. sapiens, which is essentially an incertae sedis used to avoid having to specify where Neanderthals sit in relation to modern Homo sapiens.