n. A guitar with an additional string, most often tuned to a B (one fourth below the low E string). Originally used by several jazz artists to extend their ability to chord and play basslines. Was not introduced to the mass market until the early 1990's by Steve Vai.
The instrument did not gain in popularity until bands such as Korn began using them in the mid 1990's. For this reason, the seven string guitar is often erroneously viewed as just a "nu metal" fad.
Currently, seven strings can be seen in a variety of roles in various hard rock and metal subgenres. This is often to achieve crushing lows without the loss of any solo range from detuning. Though widely accepted within these niche roles, it is rarely seen elsewhere.
A number of guitar manufacturers are currently producing this unique instrument. Ibanez, who started the trend, still turns out new models fairly regularly. Schecter has a wide variety of models to choose from. ESP and Agile (an internet distributor) offer sevens with a 27" scale length for better note clarity.
Maestro Alex Gregory: I created a seven string guitar prototype for rock music long before you, Vai.
Steve Vai: Yes, but it was never produced. Plus, it used a high string instead of a low one.
Alex: Yeah, well...I'm awesome. And I can't wait to piss on your grave.
Vai: You already did. *shows album cover*
Alex: Fuck! *walks away*