An onomatopoeic adverb meaning to smile sweetly or broadly; often used as 'にっこり笑う' to describe a gentle, pleased grin.
赤ちゃんがにっこり笑った。
The baby smiled sweetly.
店員がにっこりと笑って迎えてくれた。
The shop assistant greeted me with a warm smile.
合格の知らせを聞いて彼女はにっこりした。
She smiled broadly when she heard the news of her acceptance.