Able to be made less bright; describing lights or fixtures that can have their brightness adjusted or reduced.
From dim + -able (suffix meaning 'capable of being'). This is a modern word that became common with electric lighting technology.
Dimmable is a word that practically didn't exist before electric lights—you can't dim a candle easily with a switch, but LED bulbs made this feature so common that 'dimmable' is now printed on light bulb packages everywhere.
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