What term describes an amateur station that is transmitting and receiving on the same frequency?
Why Simplex operation means the station transmits and receives on one single frequency, so only one station can talk at a time and everyone hears each other directly without a repeater. This is the normal mode for direct radio-to-radio contacts on VHF/UHF calling frequencies such as 146.52 MHz. Repeaters, by contrast, use duplex: they receive on one frequency and retransmit on another, which is why your radio applies an offset when working through one.
Watch out Full duplex means transmitting and receiving at the same time on two different frequencies, like a telephone conversation, which is the opposite of what is described here.
Simplex = single frequency. Duplex = dual frequencies (repeater offset).