The first middleweight fight with gloves may have been between George Fulljames and Jack (Nonpareil) Dempsey (no relation to the more famous heavyweight of the same name). Dempsey knocked out Fulljames on July 30, 1884.
In the modern era, middleweight means that the fighter's offical weight does not exceed 160 pounds. (In practice, however, since weigh-ins take place usually 24 hours before the actual fight, many fighters show up at the actual ringside weighing more.)
Some notable middleweights have been: