One-night stand
A one-night stand refers to a one-time sexual encounter with someone you know only casually. The contact usually lasts only one night or a few hours and is generally not intended as a long-term relationship. The focus is typically on the sexual encounter itself, without any obligation for it to continue.
As with any sexual contact, consent, mutual respect, and clear expectations are important. In brief encounters in particular, boundaries, protection, and safer-sex considerations should be clarified consciously. The abbreviation is ONS.