A compound is a form consisting of two adjacent roots that instantiates, or was created by, a compound construction.
The definition is from Haspelmath (2025: 5).
“In [defining compounds] we do not aim for more than a rough approximation, as it is clear that the respective criteria are not only in part language-specific, but also a matter of controversial theoretical debate.” (Finkbeiner & Schlücker 2019: 9)