'Idol' in the Bible

Therefore concerning the eating of things sacrificed to idols, we know that there is no such thing as an idol in the world, and that there is no God but one.

However not all men have this knowledge; but some, being accustomed to the idol until now, eat food as if it were sacrificed to an idol; and their conscience being weak is defiled.

What do I mean then? That a thing sacrificed to idols is anything, or that an idol is anything?