A Zionist is someone who believes in the right of Jews to enforce their religious beliefs on the natives of Palestine by claiming the land of that area and forming Israel.
Pretty simple.
While you can argue that only Jews can be true Zionists, any Christian that supports the formation of Israel to help bring about the Rapture is a supporter of Zionism and can, therefore, be considered a "Zionist".
Of course, the best definition of "Zionist" is "anyone who thinks mutilating Arab babies is doing God's work according to God's plan".