Things that send out or discharge light, heat, sound, radiation, or other particles.
From Latin 'emittere' (to send out, from 'e-' out plus 'mittere' to send) plus '-er' (one that does), becoming common in scientific contexts in the 20th century.
In physics, understanding emitters led directly to radio, television, and lasers—basically, controlling what things emit became one of humanity's most transformative superpowers.
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