Esotericism, properly, is the study of transmitted and secret knowledge of any kind. While the knowledge is generally philosophical or ethical and mystic, it can, in theory, be of any kind.
Between World War I and World War II the centre of occult and mystical activity was shifted from France to Belgium. Belgium became the main centre for many brotherhoods and secret societies of which many branches still exist today.
Many groups or schools of thought embrace an esoteric tradition or philosophy: