MON according to Wikipedia:
Mixed oxides of nitrogen (MON) are solutions of nitric oxide (NO) in dinitrogen tetroxide/nitrogen dioxide (N2O4 and NO2).
The addition of a small amount of nitric oxide (1%-10%) makes the oxidizer less corrosive, but slightly less powerful as well, and changes the freezing point of the liquid. MON3 means 3% nitric oxide by mass, which is a common formulation for American engines.
From Clark's "Ignition!":
NO is an extremely effective freezing point depressant for N2O4. ... G. R. Makepeace and his associates, at NOTS, were able to show, in 1948, that 25 percent of NO would bring the freezing point of nitrogen tetroxide down below the required -65 F, and that 30 percent would depress it well below the magic -100 F. However, the vapor pressure of the latter mixture at 160 F was unacceptably high, about 300 psi.
(The freezing point of plain N2O4 is 16 F; Clark says -9 F, but this likely varies with contaminants. Military services wanted reliable storable fuels for all climates.)
Several agencies tried the mixed oxides of nitrogen (MON-25 or MON-30 or whatever, with the number designating the percentage of NO in the mix) with various fuels, and discovered that it was more difficult to get a good performance with MON than with neat nitrogen tetroxide. ... For this reason, and because of its high vapor pressure, investigators turned away from MON for some years.
In full, the usual modern hypergolic oxidizer is "an equilibrium mixture of dinitrogen tetroxide and nitrogen dioxide with an admixture of nitric oxide"; it's no wonder this gets abbreviated to "mixed oxides of nitrogen" and then to MON.