Sorcery
Sorcery is a term used to explain when a practitioner practices with multiple forms of magic rather than remain grounded to one with a high-level of proficiency. These practitioners are called Sorcerers.
Methodology
Sorcery combines everything from Shamanism to Diabolism and everything in-between, with the primary point being that they can utilize the individual fields with expertise.
Someone who uses multiple arts to lesser effect, mere bits and pieces, is a Dabbler.