The act of singing with your ipod in and turned up so that only you, the singer, has the benefit of the original musical score to cover your own voice.
Those in the vicinity of the singer are usually, though not always, subjected to a horribly distorted version of the song, wherein the lyrics are mixed up and/or slurred together, the tune is wrong, and the singer is off key.
Jane: (Singing noisily and off-key) G-L-A-M-O-R-O-U-S, we’re flying so fast, up in the sky, pop and champagne, living my life in a fast plane, I can’t change, cause I’m glamorous. I’m so foxy, foxy!
Caitlin: What the hell are you singing?
Susan: She has her ipod in. She’s singing icapella.