The word dialogue has interesting origins. Rather than what you might suspect from looking at monologue (etym. “alone + discourse”), the prefix of dialogue doesn’t indicate the number of people involved. In fact, the word for a discussion between two is a duologue.
In the word dialogue, the prefix dia- describes something else about the discussion. It’s a Greek prefix which means “through” or “across”. I don’t know what the ancient Greeks were thinking when they began using the word, but I’ve pondered two interesting possibilities.
First, a dialogue could mean a discourse or discussion through an intermediate medium. For instance, when Plato wrote his famous philosophical Dialogues, he used the medium of writing to convey his ideas. My words now are reaching you through a number of intermediate media. There’s the written word, being retrieved and transformed by your browser or aggregator, finally being rendered by your display for you to read.
The second, and more appealing, idea is that a dialogue creates a bridge across the differences between people. Dialogue is a way of working out what stands between those taking part, and breaking through those obstacles. I like that potential etymology. Kind of poetic, don’t you think?