Sideband

In amplitude modulation (AM), a sideband is a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process.

Amplitude modulation of a carrier frequency results in two sidebands. The frequencies above the carrier frequency constitute what is referred to as the upper sideband, those below the carrier frequency, constitute the lower sideband. In conventional AM transmission, the carrier and both sidebands are present. Transmission in which only one sideband is transmitted is called single-sideband transmission.

Source: from Federal Standard 1037C and from MIL-STD-188