• 蛍ほたる — firefly
• 蛍火ほたるび — firefly light; a faint glow
• 蛍光けいこう — fluorescence; fluorescent light
• 蛍光灯けいこうとう — fluorescent lamp
• 飛蛍ひけい — flying fireflies (poetic)
• 蛍雪けいせつ — diligent study; studying late into the night

This kanji belongs to the “insect” group (虫). It’s used for **fireflies**, and by extension for **light that glows** — especially in words like 蛍光.