As a child I was taught that opaque
opaque means doesn't let any light through at all, as opposed to translucent
translucent (lets some light through, but diffused/frosted) or transparent
transparent (completely clear, lets you see clear images).
However, it seems a lot of people use opaque
opaque as synonymous with translucent
translucent. Indeed if you ask Google for its definition of opaque
opaque, and looks at its definition, usage example (“opaque with steam”), and synonyms, it seems to support the translucent
translucent-synonymous usage. I don't know what Google's data source is for their definitions, but it's clearly descriptive of real usage (as most dictionaries are), not prescriptive of how a word “ought” to be used.
I've certainly heard people (mis)use opaque
opaque as synonymous with translucent
translucent in everyday speech.
So now I'm looking for a single word to use in place of opaque
opaque to firmly communicate that the thing being described is completely impenetrable to light.