one-night stand
noun /ˌwʌn naɪt ˈstænd/
  /ˌwʌn naɪt ˈstænd/
(informal)- a sexual relationship that lasts for a single night; a person that somebody has this relationship with
- I wanted it to be more than a one-night stand.
 - For her I was just a one-night stand.