A virtuoso is a performer with extraordinary technical skill, most often a musician — a violinist who plays passages nobody else can play cleanly, a pianist whose hands seem to do the impossible. The word is not just praise for being good; it points at skill of a rare, almost showy kind. Two things follow from that. First, it is usually about technique rather than feeling: you can be a virtuoso and still leave the audience cold, and people sometimes say exactly that as a criticism — "a virtuoso, but no heart." Second, it sits mostly in the world of classical music and formal performance. A rock guitarist who plays very fast may be called a virtuoso, half seriously, half as a joke about showing off.

Outside music it stretches to any field where fine skill shows: a virtuoso of the kitchen, a virtuoso chess player. That use is a little literary — you meet it in writing more than in speech.

It also works as an adjective before a noun: a virtuoso performance, virtuoso playing. Here it simply means dazzlingly skilful, and this use travels further than the noun does — people write "a virtuoso piece of writing" quite naturally.

The plural is virtuosos, or virtuosi if the writer wants to sound Italian and formal. Both are correct; virtuosos is safer.