No need to guess or try to write new definitions. Here are definitions accepted by all administrations around the world and included in the radio regulations:
RR 1.125 simplex operation: Operating method in which transmission is made possible alternately in each direction of a telecommunication channel, for example, by means of manual control2.
RR 1.126 duplex operation: Operating method in which transmission is possible simultaneously in both directions of a telecommunication channel2.
RR 1.127 semi-duplex operation: A method which is simplex operation at one end of the circuit and duplex operation at the other.2
2 1.125.1, 1.126.1 and 1.127.1 In general, duplex operation and semi-duplex operation require two frequencies in radiocommunication; simplex operation may use either one or two.
I hope this helps.
73, art….. W4ART Arlington VA