Plain English Meaning
The official test you take with an examiner to earn a pilot certificate or rating. It has a ground (oral) part and a flying part, and you must show you can both talk through and actually perform the required tasks to FAA standards.
Full Definition
An FAA evaluation, conducted by a designated examiner or FAA inspector, in which an applicant demonstrates the knowledge, risk management, and flight skills required to be issued a pilot certificate or rating. It consists of an oral portion and a flight portion, and is conducted in accordance with the appropriate Airman Certification Standards (ACS) or Practical Test Standards (PTS).