Literally "the worship of idols," it is giving divine honors to a creature. In the Decalogue it is part of the first commandment of God, in which Yahweh tells the people, "You shall have no gods except me. You shall not make yourself a carved image (Greek eidolon, idol] or any likeness of anything in heaven or on earth or in the waters under the earth; you shall not bow down to them or serve them" (Exodus 20:4-5).
The early Christians were martyred for refusing to worship idols, even externally, but practical idolatry is a perennial threat to the worship of the one true God. Modern secularism is a form of practical idolatry, which claims to give man "freedom to be an end unto himself, the sole artisan and creator of his own history." Such freedom, it is said, "cannot be reconciled with the affirmation of a Lord who is author and purpose of all things," or at least that this freedom "makes such an affirmation altogether superfluous" (Second Vatican Council, Constitution on the Church, 51).
Idolatry is always gravely sinful. Even under threat of death and without interiorly believing in the idol, a Christian may not give divine honors to a creature, thereby violating the duty of professing faith in God.