The act of someone giving or receiving their first kiss, or a particular couple kissing for the first time.
A first kiss is exactly what it sounds like. A person's very first kiss, or the first kiss between a couple falls under this concept.
A person's first kiss is generally a bigger deal in Japanese culture than it is in western culture, which is only exaggerated in anime and manga, to the point where it's virtually a sacred act in some cases. In some cases though, accidental first kisses occur, often between members of the same sex for comedic effect.