Socius is the Latin for ally; specifically, allies of Rome. The plural of socius is socii. As Rome conquered some cities the Italian peninsula and made peacetime alliances with others, it formed treaties (foedera) and made allies. Although the socius was not officially a subject of Rome, the arrangements favored Rome, especially in the cases where the treaty was not voluntary. By the Punic War Rome had made 150 separate treaties with its allies.
The early 1st century B.C. war known as the Social War was fought between Rome and its socii.
- A. N. Sherwin-White , Tim J. Cornell "socii" The Oxford Classical Dictionary.