In Sergei Lukyanenko's Watch series, there are several different "grades" of magical power, from seventh-grade all the way up to first-grade and then beyond. As I understand it, everyone above first-grade power is called a Higher Magician (or Higher Vampire, etc.): they are "Higher Ones".
But another title I've seen used now and then is "Great One". Gesar and Zabulon are sometimes addressed or referred to as Great Ones, and I think so is Foma/Thomas the Rhymer in Edinburgh, Rustam in Uzbekistan, and a very few others. But when
Anton becomes a Higher One
at the end of Twilight Watch, nobody in Last Watch ever refers to him as "Great One".
What is the difference between a Higher One and a Great One? Is "Great One" a loosely defined term of respect for extremely experienced and high-ranking Higher Ones? Does it mean something not directly related to magical power? (Do the original Russian terms make anything clearer?)