Wiktionary
subbasin
n. (context geography English) A basin within a larger basin.
Wikipedia
Subbasin
A subbasin is a structural geologic feature where a basin forms within a larger basin. It is sometimes spelled "sub-basin" depending on an author's preference, but the predominant form is "subbasin". Some subbasins are referred to as "sags", although that term has a broader definition than a subbasin does.
The term subbasin has common use in geologic literature, but has yet to be included in the API Glossary of Geology.