Which term refers to someone who practices beauty treatments and services under regulation?

The term that accurately refers to someone who practices beauty treatments and services under regulation is "Cosmetologist."

A cosmetologist is a licensed professional trained in various beauty services including hair, skin, and nails, and is subject to state regulations that govern their practice. This licensure ensures that cosmetologists have met educational requirements and adhere to health and safety standards, thereby protecting clients and maintaining industry integrity.

While "beautician" and "practitioner" might refer to individuals providing beauty services, the terms do not specifically imply the standardized training and regulatory compliance inherent in the role of a cosmetologist. An "apprentice" is someone who is in training and has not yet obtained full licensure to practice independently.