Caveat

Etymology

From Latin caveat, “let him beware,” from cavēre, “to be on guard.” It entered English first as a legal term for a formal notice halting a proceeding until the filer is heard; the word is the bare Latin verb, and its “beware” force survives in the modern qualifying caveat.

noun · /KAV-ee-at/
A warning or qualification that limits or conditions a statement.
What it names
The “but note this” attached to a claim — the explicit qualification that keeps a statement from being taken as broader or surer than intended.
Example
The results look promising, with one caveat: the sample was far too small to trust.
Related idea
A caveat warns you of a limitation; a proviso sets a formal condition that must be met.