Tzaddik means righteous. In Hasidism the tzaddik or rebbe is a holy leader who lives in devekut and mediates between his followers and God, praying for them and guiding them. Dynasties of rebbes formed around great tzaddikim.
Tzaddik
The righteous one: in Hasidism, the spiritual leader through whom divine blessing flows to the community.
Also: Tzadik, rebbe