Also see definition of "Root" in Word Study
Study Dictionary
Root
ROOT [ISBE]
ROOT - root (shoresh; rhiza): Frequently mentioned in the Old Testament and New Testament, but almost always in a figurative sense, e.g. "root of the righteous" (Prov 12:3,12); "root that beareth gall" (Dt 29:18); "Their root shall be as rottenness" (Isa 5:24); "root of bitterness" (Heb 12:15). Also of peoples: "they whose root is in Amalek" (Jdg 5:14); of Assyria (Ezek 31:7); "Ephraim is smitten, their root is dried up" (Hos 9:16); "Judah shall again take root downward" (2 Ki 19:30; compare Isa 27:6; 37:31); the root of Jesse (Isa 11:10; Rom 15:12); root of David (Rev 5:5; 22:16).
Also see definition of "Root" in Word Study