Chasidei Umos HaOlam is the Hebrew term used by Maimonides in the Laws of Kings 8:11. It is generally translated as "Righteous Gentile". They are the "Observant Noahites". The term indicates non-Jews who have taken upon themselves the observance of Noahide Law, and do this because it was revealed through Moshe Rabbenu as explained by Jewish tradition. According to Maimonides, Chasidei Umos HaOlam are assured a portion in the world to come. This is something that is not assured for the Chochmei Umos HaOlam who observe Noahide Law, but do it for their own rational reasons.
When the kingdom is established, the Chasidei Umos HOlam will become Tzadikei Umos HaOlam.
The Arabic sources refer to them as מו֞מן.