Why Amplitude modulation varies the strength of the RF signal with the audio. Single sideband is simply AM that has had the carrier and one of the two sidebands filtered out, leaving only the information-bearing sideband, so it is still a member of the AM family. Removing the carrier and redundant sideband cuts the bandwidth roughly in half and puts all the transmitter power into the useful signal.
Watch out Phase shift keying varies phase, not amplitude, and spread spectrum is a technique for spreading energy across a wide band; packet radio is a data protocol, usually sent over an FM transmitter, not a modulation type in itself.
SSB = AM minus carrier minus one sideband. If the name has 'sideband' in it, think amplitude.