noun, noun or participle which takes 'suru'
Often used in set phrases like 一笑に付す (to laugh something off) or 一笑を買う (to get a laugh). As a standalone noun, it refers to a single laugh or smile.
彼の冗談に思わず一笑した。
I couldn't help but laugh at his joke.
その提案は一
The proposal was laughed off.