Easton's Bible Dictionary

Fornication - in every form of it was sternly condemned by the Mosaic law (Lev. 21:9; 19:29; Deut. 22:20, 21, 23-29; 23:18; Ex. 22:16). (See ADULTERY.)

But this word is more frequently used in a symbolical than in its ordinary sense. It frequently means a forsaking of God or a following after idols (Isa. 1:2; Jer. 2:20; Ezek. 16; Hos. 1:2; 2:1-5; Jer. 3:8,9).