eidōlon

In everyday Greek this word meant a likeness or an appearance: a reflection, a picture in the mind, even a ghost. The Jewish translators of the Hebrew scriptures took it over as their standard label for the gods of the surrounding nations and for the statues those gods were worshipped through, and by the time the New Testament was written that religious use is the only one it has.

Inside that religious use the word points two ways, and the New Testament moves between them without warning. Sometimes it names the object you could touch: gold, silver, bronze, stone, or wood, shaped by hand and unable to see or hear. Sometimes it names the god itself, the being worshippers believed stood behind the object. The two run together because ancient worshippers did not separate them. The figure was not a picture of the god; it was where the god was met. So Paul can say flatly that an idol "is nothing in the world," meaning no such god exists, only pages after John's vision describes idols by the metal they are made of.

The word is never a neutral description. The ordinary Greek word for an image or likeness (εἰκών) can be used of a portrait on a coin or of Christ as the image of God, but to call something an idol is already to say that what is worshipped there is a counterfeit. That edge is why the plural can stand in for a whole religion rather than a set of objects: when the Thessalonians "turned to God from idols," they left a way of life, not a shelf of statues.