A sign, feeling, trace, or touch — used broadly to indicate mood, physical condition, or a sense/indication of something (e.g., 'I feel like I might be getting a cold').
朝から頭が重くて、風邪の気がする。
My head has felt heavy since morning; I think I might be coming down with a cold.
彼の顔に疲れの気が表れていた。
Tiredness showed on his face.
部屋に湿気がこもっている気がする。
I feel like the room is stuffy with humidity.
事件の気配を感じて、警察に連絡した。
Sensing that something was off about the incident, I contacted the police.
Used in expressions conveying 'seemingly' or 'for some reason' (e.g., 気のせい) — a prefix-like or idiomatic use that indicates that something appears or feels a certain way.
気のせいか、今日は電車がいつもより空いている気がする。
Maybe it's just my impression, but the train seems emptier than usual today.
気を張っていたからか、家に帰った途端にほっとした気持ちになった。
Maybe because I had been on edge, I felt relieved the moment I got home.