A Fleet Replacement Squadron (FRS), is a unit of the United States Navy or Marine Corps that trains Naval Aviators, Naval Flight Officers (NFOs) and enlisted Naval Aircrewmen on the specific front-line aircraft they have been assigned to fly. Students, referred to as Replacement Pilots, Replacement Flight Officers or Replacement Aircrew are either newly winged aviators, aviators transitioning from one type aircraft to another, or aviators returning to the cockpit after a period of non-flying. After completing the training regimen, graduates are assigned to fleet squadrons. Additionally, FRSs are responsible for training aircraft mechanics, providing replacement aircraft for fleet squadron attrition, and standardizing maintenance and aircraft operations.

A naval aviator is a commissioned officer or warrant officer qualified as a crewed aircraft pilot in the United States Navy or United States Marine Corps. United States Coast Guard crewed aircraft pilots are officially designated as "Coast Guard aviators", although they complete the same undergraduate flight training as Navy and Marine Corps crewed aircraft pilots, and are awarded the same aviation breast insignia.