It is just a term which got established. Most of the times, I just talk about the 35mm format. The best explanation is, that Canon and Nikon used to sell digital versions of their 35mm camera with smaller sensors. And when they started to make 35mm digital sensors, these cameras were "full frame", as the digital camera had the same sensor size as the film cameras of the same system had. I don't care to much about those terms, I am happy with Olympus.
With respect to the Leica: if you can trust your self control, by all means try out a Leica, it is an experience one should have. But yes, deposit your credit card at a safe place, they are lovely. For me the downfall was that I bought a M-mount Voigtländer 40/1.4 as a short tele lens for mFT, back then when there where not many prime lenses to choose from. It works lovely on a digital M :)
With respect to the Leica: if you can trust your self control, by all means try out a Leica, it is an experience one should have. But yes, deposit your credit card at a safe place, they are lovely. For me the downfall was that I bought a M-mount Voigtländer 40/1.4 as a short tele lens for mFT, back then when there where not many prime lenses to choose from. It works lovely on a digital M :)