AM is short for amplitude modulation, which refers to the means of encoding the audio signal on the carrier frequency. In many countries, AM radio stations are known as "mediumwave" stations. They are also sometimes referred to as "standard broadcast stations" because AM was the first form used to transmit broadcast radio signals to the public. Many AM stations are either daytime-only stations or stations authorized to operate at very low power levels at night as a result of “sky wave” propagation, which occurs when the AM signal is reflected off the ionosphere and back to the earth.