There's no official definition, but to chasers I think they've generally defined one as one that's wider than it is tall.
Wider than it is tall? ARGHHHHHHHHHHHHHH & YIKES!!!! Dang, I hate tornadoes. We're under a tornado watch again tonight, but so far, most of the nasty weather is north of us. I'm near the St. Louis area. Last night the twisters were south of us..........thank God! I hope the folks involved with this Wedge Tornado made it through okay.