A Great Clear is accomplished by making a Katamari far larger than the basic goal, but not large enough to receive a Super Clear. Depending on the game of the series, the Great Clear will either be a star with 7 points (in Katamari Damacy) or will have a glowing halo around the planet or star (in later games).
Generally, a Great Clear can be accomplished by achieving a size larger than halfway between a Normal Clear and a Super Clear. However, this is not consistant and can be anywhere from just over the normal goal or just slightly less than the Super Clear goal.
The terms Normal Clear, Great Clear, and Super Clear were not originally created by Namco or the Katamari development team. Rather, they were invented by fans of the game and have since become semi-official. Also, since no players guides were ever printed for the games in the Katamari series, there has been little opportunity for such terms to be coined in a way that players could pick up on, except among themselves.