This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
A near field monitor is one that is design to be listened to in the
near field. Simple, eh?
The "near field" of a loudspeaker is area where the direct,
unreflected sound from the speaker dominates significantly over the
indirect and reflected sound, sound bouncing off walls, floors,
ceilings, the console. Monitoring in the near field can be useful
because the influence of the room on the sound is minimized.
Near field monitors have to be physically rather small, because you
essentially need a small relative sound source to listen to (imagine
sitting two feet away from an 18" woofer and a large multi- cellular
horn!). The physics of loudspeakers puts severe constraints on the
efficiency, power capabilities and low frequency response of small
boxes, so these small, near-field monitors can be inefficient and not
have the lowest octave of bass and not play ungodly loud. [Dick]
 
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