A crossover artist generally refers to an artist who has had substantial success in multiple genres of music. It’s been used particularly to refer to a handful who have marketed themselves as performers of the classical and pop repertoire.
One can make the argument, however, that most of them are simply pop artists who try to sing in a classically trained style.
Going by its definition, or at least by its generally agreed notion, a crossover artist must be someone who, let’s say, has professionally performed operatic roles or oratorio solos while concurrently rocking pop music establishments.