14 CFR § 135.321 - Applicability and terms used.
(a) Except as provided in § 135.3, this subpart prescribes the requirements applicable to -
(1) A certificate holder under this part which contracts with, or otherwise arranges to use the services of a training center certificated under part 142 to perform training, testing, and checking functions;
(2) Each certificate holder for establishing and maintaining an approved training program for crewmembers, check airmen and instructors, and other operations personnel employed or used by that certificate holder; and
(b) For the purposes of this subpart, the following terms and definitions apply:
(4) Differences training. The training required for crewmembers who have qualified and served on a particular type aircraft, when the Administrator finds differences training is necessary before a crewmember serves in the same capacity on a particular variation of that aircraft.
(5) Recurrent training. The training required for crewmembers to remain adequately trained and currently proficient for each aircraft, crewmember position, and type of operation in which the crewmember serves.
(6) In flight. The maneuvers, procedures, or functions that must be conducted in the aircraft.
(7) Training center. An organization governed by the applicable requirements of part 142 of this chapter that conducts training, testing, and checking under contract or other arrangement to certificate holders subject to the requirements of this part.
(8) Requalification training. The training required for crewmembers previously trained and qualified, but who have become unqualified due to not having met within the required period the -
(i) Recurrent pilot testing requirements of § 135.293;
(iii) Line checks required by § 135.299.