The "Define Dancing" scene serves a dual purpose, half for the Captain and half for WALL-E and EVE. Both serve the same purpose, in that the characters involved are learning about being human. Humanity is one of those tropes sci-fi writers love to work with, robots especially dancing on the line between human and inhuman (Star Trek's Data, Futurama's Bender, and the Warforged of Dungeons and Dragons). WALL-E is no exception, playing with the idea of humanity with its robots.

As of the Define Dancing scene, the humans aboard the Axiom have had their existence automated for centuries. Entire generations have lived and died onboard without seeing, or even being taught about Earth. The babies seen in the nursery are being taught about the alphabet through Buy-N-Large's corporate filter. In a sense, they have had their humanity stripped away.

WALL-E and EVE are explicitly inhuman, being robots; but WALL-E acts very humanlike, collecting human memorabilia and artifacts. EVE begins as entirely inhuman and robotic, but learns to act humanlike through her interactions with WALL-E and completion of her directive. She inevitably rejects her main directive to help WALL-E, and eventually repairs him, only to see that she reset him. The humanity of these two robots is fluid.

The Define Dancing scene, in which WALL-E and EVE dance in space, is the pinnacle of them both experiencing humanity. The Captain, in parallel, is going through definitions and examples of human culture, and gets from "Earth" to "hoedown", and inevitably, "dancing". Both parties are learning about being human - the Homo sapiens just beginning, and the inhumans at the apex of their journey.