Easton's 1897 Bible Dictionary:

Zadok
   righteous. (1.) A son of Ahitub, of the line of Eleazer (2 Sam.
   8:17; 1 Chr. 24:3), high priest in the time of David (2 Sam.
   20:25) and Solomon (1 Kings 4:4). He is first mentioned as
   coming to take part with David at Hebron (1 Chr. 12:27, 28). He
   was probably on this account made ruler over the Aaronites
   (27:17). Zadok and Abiathar acted as high priests on several
   important occasions (1 Chr. 15:11; 2 Sam. 15:24-29, 35, 36); but
   when Adonijah endeavoured to secure the throne, Abiathar went
   with him, and therefore Solomon "thrust him out from being high
   priest," and Zadok, remaining faithful to David, became high
   priest alone (1 Kings 2:27, 35; 1 Chr. 29:22). In him the line
   of Phinehas resumed the dignity, and held it till the fall of
   Jerusalem. He was succeeded in his sacred office by his son
   Azariah (1 Kings 4:2; comp. 1 Chr. 6:3-9).

     (2.) The father of Jerusha, who was wife of King Uzziah, and
   mother of King Jotham (2 Kings 15:33; 2 Chr. 27:1).

     (3.) "The scribe" set over the treasuries of the temple by
   Nehemiah along with a priest and a Levite (Neh. 13:13).

     (4.) The sons of Baana, one of those who assisted in
   rebuilding the wall of Jerusalem (Neh. 3:4).


Hitchcock's Bible Names Dictionary (late 1800's):

Zadok, just; justified