"Automatic Teleprinting Over Radio"; popular digital mode on HF
AMateur Teletype Over Radio. An advanced form of RTTY usually operated on the HF bands.
AMTOR is a specialised form of RTTY protocol. The term is an acronym for AMateur Teleprinting Over Radio and is derived from the commercial SITOR system (Simplex Telex Over radio) developed primarily for maritime use in the 1970s.