What exactly does it mean to be the undisputed champion?

[Hydro]

Sometimes people argue over it even when a guy has all 3 belts.

Look at Tyson in the late 80s before he beat Spinks.

He was called the undisputed champ, but Spinks was still linear and The Ring called him the champ before he got maimed by Tyson.

[Fighting Cougar]

800 comment! I thought it was just to get all the big belts. Like Hopkins and Judah when they got the Major belts. WBC WBA and IBF.