The funny thing is, today the term crossover seems a vocabulary convenience, a casual catchall, really, for a singer (or musician or artist of an sort, I'd suppose) who moves across musical styles. In terms of other musical genre--in American music, for example, we also read of country-western singers who "crossover" to rock or pop music--in fact, there's the whole argument from die-hard country fans that an enormous chunks of country music today doesn't represent an historic musical evolution at all, but a massive shapeshift, a crossover, into pop
Among classical artists, though, it doesn't seem as if virtuosity across different musical genres is anything new--and among those whom we've had recorded--well, Caruso hit whatever the charts were in his day with the World War I icon, "Over There," among others. I'm guessing that the cultural connotations associated with crossover that divided classical from pop are blurry in today's musical environment. But if there's to be a definition, I kind of like Renee Fleming's in the New York Times article from the thread Derek's linked to--indefinite, but it gives something to hang onto:
for the most part crossover refers to classical artists who claim pop
pieces and perform them in essentially a classical manner.
In which case, it's an awfully wide field--and all the better for us as listeners. And here's a fun one--a dual crossover?
Freddy Mercury and Monserrat Caballe with "Barcelona." Best, Lee Ann