I've always thought of it more as "Co-Pilot", but formally: "Autopilot" might truly be the better definition (lane-keeping, distance-keeping), whereas a "Co-Pilot" (in aviation) implies more active control, ie: pulling you up from a nose dive.

So... informally, "Tesla Co-Pilot" => "You're still the pilot but you have a helper", vs "Tesla Autopilot" => "Whelp, guess I can wash my hands and walk away b/c it's AuToMaTiC!"

...it's tough messaging for sure, especially putting these powertools into peoples hands with no formal training required. Woulda-coulda-shoulda, similar to the 737MAX crashes, should "pilots" of Teslas required training in the safety and navigation systems before they were "licensed" to use them?