first kiss
Commonly used in casual conversation and romantic contexts. Synonymous with ファーストキス, but 初キス has a slightly more native Japanese feel.
See also: ファーストキス
When was your first kiss?
I still remember my first kiss with her.
Compound of 初 (hatsu, 'first') and キス (kisu, 'kiss'). The katakana portion is a loanword from English.