Have to be interesting to watch and listen to both from their mouth and the piano keys. Lots of technical masters and emotional psychos at the piano all over the world, but they are not regarded virtuosos because they 1) can't combine the two, technique and emotion to a maximum. 2) have no communication with the audience. Although i do find it sad there are many examples of technical masters who have spurts of emotion in their playing and 0 audience communication and they are famous ,sell out concerts and are called virtuoso!
Why I say you need an audience? Because a real virtuoso can master their instrument in any given environment. Those that hide away are not practicing their art out of their bubble, hence are not really a complete virtuoso.
But I would think that the meaning virtuoso is flexible and can come in different disguses. Those that everyone knows, and the quiet achievers. There are a huge amount of virtuoso who cannot communicate with an audience, they are petrified about audience, or they just don't care for them at all. But I would have to say all virtuoso can absorb music at an unnatrual rate, and play any wild technical passage as if it where honey on the fingers and memorise and sight read both at a near maximum level.