Iota means a tiny amount, the smallest possible bit of something. When you say there isn't an iota of truth in a rumor, you mean there's absolutely no truth to it at all, not even the tiniest speck. If your room hasn't changed one iota since yesterday, it looks exactly the same.

The word comes from iota, the ninth letter of the Greek alphabet, which looks like this: ι. It's the smallest Greek letter, just a simple line, which is why it came to mean something extremely small.

You'll often hear iota in phrases like “not one iota” or “without an iota of doubt.” A detective might say there isn't an iota of evidence linking the suspect to the crime. A teacher might tell a class that she won't tolerate even an iota of disruption during the guest speaker's visit.

The word emphasizes smallness to make a point. Instead of simply saying “no difference,” saying “not one iota of difference” stresses how completely identical two things are. It's a way of being emphatic and precise at the same time.