In Jewish tradition, tzaddik is more than just a generic word for a righteous individual. It is a title of honor for a person of exemplary righteousness, who plays a specific role in the community as such:
The word " tzaddik" literally means "righteous one." The term refers to a completely righteous individual, and generally indicates that the person has spiritual or mystical power. A tzaddik is not necessarily a rebbe or a rabbi, but the rebbe of a Chasidic community is considered to be a tzaddik. (From Judaism 101)In the Psalms we find tzaddik in the singular (many translations fail to reflect this) -- the righteous man, rather than the righteous ones or the righteous people, or even the righteous individuals. Christologically speaking, the tzaddik in the Psalms is not just any righteous man, but the Righteous Man, the very embodiment of the Law, the Word incarnate. Read a few Psalms with this in mind, and the prophetic and the Christological leap off the page. Then read again remembering that in baptism we are united with this Tzaddik in His sufferings and triumph, and that He has placed His righteousness in us.