Wiktionary
foederatus
n. A confederate. One of the tribes bound by treaty, who were neither Roman colony nor had they been granted Roman citizenship but were expected to provide a contingent of fighting men when trouble arose.
Usage examples of "foederatus".
A new Emperor, Theodosius, made peace with the Goths in 381, and most of their warriors entered the Roman service as foederati: allies, we'd say.