I have heard the two terms thrown around in a somewhat interchangeable manner it seems. Are there any major differences? I haven't gathered that they have a different meaning, but I don't understand why there would be two different words if that were the case, as it seems authors choose one over the other somewhat randomly.
They seem to be synonymous: "Monostatic radar is the term given to a radar in which the transmitter and receiver are collocated. This is the conventional configuration for a radar, but the term is used to distinguish it from a bistatic radar or multistatic radar." (https://en.wikipedia.org/wiki/Monostatic_radar)