Definition and assumption

Hello Rabbi,
What is the difference between a definition and a postulate? On page 114 of the first book of the trilogy: "He does not really assume the existence of God. He assumes a definition of God as a being possessing all perfections…but definitions are not postulates." Kant attacks Anselm for assuming existence in advance, and this is what follows from the definition of the concept of God. Clearly, this is the desired postulate, since it is a logical argument. I did not understand what the problem was and what the difference was between defining God as a perfect being that supposedly includes his existence and implicitly assuming that he exists.
I hope I understood correctly, thank you very much!