A tubular fluorescent lamp (fluorescent light) used for indoor lighting.
部屋の蛍光灯を替えたら、明るくなった。
When I replaced the fluorescent lamp in the room, it became brighter.
蛍光灯のちらつきが気になる。
The flicker of the fluorescent lamp bothers me.
最近はLEDに比べて蛍光灯の使用が減っている。
Recently, the use of fluorescent lamps has decreased compared to LEDs.
(dated slang) A person who is slow to react; someone slow on the uptake (dated usage).
昔は反応の鈍い人を「蛍光灯」と呼ぶことがあった（古い言い回し）。
In the past, people who were slow to react were sometimes called '蛍光灯' (an old expression).