Definitions for "Magnoliidae"
a group of families of trees and shrubs and herbs having well-developed perianths and apocarpous ovaries and generally regarded as the most primitive extant flowering plants; contains 36 families including Magnoliaceae and Ranunculaceae; sometimes classified as a superorder.
Magnoliidae is a botanical name of a subclass. The of the subclass will vary with the being used (there are many such systems). The only requirement is that it must include the family Magnoliaceae (Art 16 of the ICBN).