taru-adjective, adverb taking the 'to' particle
brilliantly shining; glittering
Literary or poetic word describing light that shines brilliantly or glitteringly. Used as a taru-adjective or adverb with と. Rare in modern everyday speech.
星が輝々と夜空に瞬いていた。
The stars were glittering brilliantly in the night sky.
彼女の瞳は輝々として、まるで宝石のようだった。
Her eyes were shining brilliantly, just like jewels.
Common mimetic adverb for sparkling or glittering. 輝々 is more formal and literary, emphasizing brilliance rather than playful sparkle.
Reduplication of the kanji 輝 (ki, 'shine'), forming a literary adjective/adverb. The exact historical derivation is uncertain, but it follows a pattern of reduplicated kanji compounds used in classical Japanese.