I agree that Wikipedia's definition is best, although much of it is marketing, even from early times. I was lucky enough to pick up a pair of JBL 4320 Studio Monitors recently, the precursor to the 4310's, 4311's, and so on. They are excellent speakers. "The JBL 4320 series studio monitor was introduced through Capitol Records in Hollywood and became the standard monitor worldwide for its parent company, EMI. JBL's introduction to rock and roll music came via the adoption of the D130 loudspeaker by Leo Fender's Fender Guitar company as the ideal driver for electric guitars." (Wikipedia) -good marketing examples on JBL's behalf. They were advertised as having a particularly flat freq. response, and a wide freq. response range. These were designed to compete against the widely popular Altec Lansing monitors of the time, and ended up overtaking them in overall use.
I also own a nice pair of Yamaha NS-10M's, the black "Studio" version. I challenge anyone to listen to the same recording through both pairs of "Studio Monitors", and prefer the Yamaha's. The sound is very different, and built for entirely different enviornments and listening levels. I can listen to the JBL's all day long, they sound good with anything. The Yamaha's don't get much use, as I find them harsh with vinyl, though they're tolerable at low levels. My only suggestion is to try as many speakers as possible and keep what you like. Besides, its part of the fun.