What Does "DS" Stand for in Nintendo DS?

"DS" stands for "Dual Screen." It refers to the console's two screens (top: display, bottom: resistive touchscreen)—the defining hardware feature that differentiated it from Game Boy Advance and competitors.

There is no "E" or "U" in the acronym. Model suffixes like DSi ("i" = individual, interactive, internet, camera) and DS Lite refer to revisions, not the base acronym.