The upper or lower sideband of a carrier frequency that is amplitude modulated. To save power, the carrier and one of the sidebands is eliminated, leaving only a single sideband for transmission.

Unfortunately, multiplication using PLLs compromises signal integrity, frequency resolution, and agility. Also, upconverting a DSB (double sideband) signal to single-sideband at a higher frequency, ...

The most complicated part of this communication system is the single-sideband (SSB ... which is arguably the most fun HF band. The marine net is on 14.300. Lots of DX during the daytime hours.