Something that is only one's imagination; a perception that isn't actually true (used when one may be mistaken).
さっき誰かに呼ばれた気がしたけど、多分気のせいだ。
I thought someone called me earlier, but it's probably just my imagination.
彼は私の顔を見て笑った。気のせいかな。
He looked at me and laughed. Maybe I'm imagining it.
夜道で物音がしたのは気のせいだった。
The noise on the street at night was just my imagination.