What term describes an amateur station that is transmitting and receiving on the same frequency?

Simplex communication occurs when an amateur station transmits and receives on the exact same frequency. This is the most basic and common form of direct, station-to-station radio communication, typically used when a repeater is not needed or available. In contrast, 'duplex' (or full duplex) involves using two different frequencies at the same time, which is commonly how operators interact with repeaters (transmitting on one frequency and receiving on another). 'Multiplex' and 'diplex' refer to combining multiple signals or using special filters to share antennas, which does not describe the basic act of transmitting and receiving on a single frequency.