air guitar 
noun informal
An imaginary guitar ‘played’ by someone miming to rock music: we like our audiences to sing along and play air guitar...
- He was in the middle of doing the guitar solo on his air guitar when he finally looked up for a split second and noticed me.
- So while fans will undoubtedly lap it up, while bouncing around the bedroom to an accompanying air guitar, there isn't much to recommend the uninitiated.
- Now here's something that's sure to have you crashing around the front room practicing your best air guitar moves.