Air — the gaseous mixture in which we breathe; the physical atmosphere or ambient air.
窓を開けて新鮮な空気を入れよう。
Open the window to let in fresh air.
高い山では空気が薄くなるので注意が必要だ。
Be careful on high mountains because the air becomes thin.
この部屋は空気がこもっているから、換気したほうがいい。
The air is stale in this room; you should ventilate it.
The mood or atmosphere in a situation; the social vibe or the implicit expectations of a group (as in 'reading the air').
会議で発言する前に空気を読んだほうがいい。
You should read the room before speaking in a meeting.
彼の冗談は場の空気に合わず、しらけてしまった。
His joke didn't fit the mood and killed the atmosphere.
あの店は落ち着いた空気があって居心地がいい。
That shop has a calm atmosphere and is comfortable.
A colloquial expression meaning someone with no presence — someone who doesn't stand out or seems invisible in a group.
彼は会議ではいつも空気なので、意見を求められることがない。
He's always invisible at meetings, so he's never asked for his opinion.
飲み会で一人だけ話に入れず、空気になってしまった。
At the party, one person couldn't join the conversation and became like air (i.e., invisible).