Studio monitor
A loudspeaker built to expose the flaws in a mix rather than flatter it.
The goal is the flattest frequency response achievable, so that what sounds too bassy actually is too bassy. Most modern monitors are active: amplifiers and crossover live in the cabinet, one power amp per driver, with the crossover ahead of amplification rather than after it. Near-field monitors sit a metre or so from the listener so that direct sound dominates the room's reflections.
The most famous one is, paradoxically, not very flat at all. The Yamaha NS-10 from 1978 was meant as a hi-fi speaker, sounded harsh and lean in the bass, and still became the fixture of every studio in the 1980s — on the theory that a mix that works on those works anywhere. Finland's Genelec, founded the same year, took the measurable route instead.