adverb
with a smile; pleasantly smiling
Describes a brief, cheerful smile, often a single flash of a grin. Related to ニコニコ, which implies a sustained smile.
See also: ニコニコ
彼女は私を見てニコッと笑った。
She looked at me and smiled pleasantly.
赤ちゃんがニコッとした。
The baby gave a little smile.