An audiologist is a trained professional who measures hearing loss and can fit hearing aids. An audiologist has at least a Masters degree specializing in hearing loss, and many now also have an AuD (Doctorate) degree, too. Some may even have a PhD.
An audiologist is also trained and certified to perform other evaluations and treatments such as for balance disorders.
Some audiologists work in research or educational fields, and others work with patients (typically in a clinic, an ENT office or in their own private practice) measuring hearing loss, fitting hearing aids and performing other related tests.
Those certified to work in clinical situations with patients, will have a title of "Certified Clinical Competence Audiologist (CCCA) after their names. In addition to their Masters degree, they have completed a year (called a "Clinical Fellowship Year", or CFY) of practical experience working with a practicing audiologist.
Only two types of people are authorized to measure hearing loss and fit and dispense hearing aids in the US. An audiologist is one of those two types. The other type is a Hearing Aid Dispenser (or Hearing Aid Specialist).