On the later Platonic dialogue (ca. 360 BC). What is a sophist? Historically, these were foreign teachers in Ancient Greece who taught young people the tools of philosophy and rhetoric, among other things, and espeically they claimed to teach virtue.
In this dialogue, "the Eleatic Stranger" (i.e., not Socrates, who is present but wholly silent after the first couple of pages) is trying to figure out what a sophist really is, and in the process is showing us a new procedure for defining a word, which he calls the method of division. This is sort of like 20 questions, where you start with "animal, vegetable, or mineral?" The difference, of course, being that in 20 questions, one player already knows the answer, and you might ask whether it makes any sense at all to use such a method when you really don't know what the thing is that you're looking for. If you don't know what a gastopod is, then you can't start to answer that by asking yourself what general type of thing it is. But with "sophist," or any other term (e.g., justice, knowledge) that Plato concerns himself with, of course you know enough to start the inquiry, and the point is to get more philosophically precise. So a better example might be "what is a whale," where you start with the animal you can see and end up classifying it as mammal as opposed to fish.
The Stranger ends up classifying the sophist in several ways corresponding to various properties ascribed to sophists (Is he someone who hunts for the souls and money of young people? Is he someone who divides people from their confused beliefs? Is he someone who creates false beliefs in people's heads?). This whole interest in sophistry is certainly still relevant to discourse today!
Buy the book or follow along in this online version. Dylan's fancy new Eva Brann translation is Plato : Sophist: The Professor of Wisdom (Focus Philosophical Library).
Plato picture by Genevieve Arnold.