adverb taking the 'to' particle, taru-adjective
laughing loudly; roaring with laughter
Literary or formal adverb describing a loud, unrestrained laugh. Often used with 笑う or similar verbs.
彼の冗談に、会場は哄然と笑った。
At his joke, the audience roared with laughter.
哄然たる笑い声が部屋中に響いた。
Loud laughter echoed throughout the room.
大笑い is a common noun/verb for 'big laugh', while 哄然 is a literary adverb describing the manner of laughing loudly.
爆笑 is a common noun/verb for 'burst of laughter', whereas 哄然 is a more formal, literary expression.
Compound of 哄 (loud laughter) and 然 (state or manner). The exact historical derivation is uncertain, but the spelling is conventionally associated with loud, unrestrained laughter.