The worship of idols, images, or anything made by hands, or which is not God. "Idolatry is of two kinds; the worship of images, statues, pictures, etc., made by hands; and the worship of the heavenly bodies, the sun, moon, and stars, or of demons, men, and animals.(Encyclopedia)
Webster's 1828
Also, excessive attachment or veneration for anything, or that which borders on adoration or worship.

...Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and convetousness, which is idolatry...
Colossians 3:5