I am reading an article about Minkowski space (as a vector space, which is why I am putting my question in this rubric) which is poorly translated from the Russian, and have come across several notational curiosities, most of which I have been able to figure out. However, there is one that I do not know. Talking about positive and negative radii, the author refers to "m-m circles". (Apparently "m" is not significant, as he also refers to "n-n circles" and other letters.) Specifically, he says: "The invariant of the [Lorentz] transformations is -x2+τ2 = R2 = inv ... For R2>0 the orbits are n-n Minkowski circles with real radii R, for R2<0 the orbits are m-m circles with imaginary radii Ri." Nowhere in the paper does the author indicate that he has coined these terms; apparently they are standard terms in Russia. But I don't know any Russian mathematicians or physicists. So: (1) what do these terms refer to? (guesses also welcome) (2) are these standard terms in English? If not, (3) what are the corresponding standard terms in English? Even a partial answer will be much appreciated. Thanks.