Breath; breathing—the air taken in or expelled, or the act of inhaling/exhaling.
深く息を吸ってから話してください。
Please take a deep breath before you speak.
階段を駆け上って息が切れた。
I ran up the stairs and became out of breath.
ヨガでは呼吸（息）を整えることが大切だ。
In yoga, regulating your breath is important.
Harmony or rapport—when people's movements or actions are well-coordinated (as in 'to be in sync').
このコンビは息が合っていて、漫才も息がぴったりだ。
This comedy duo is in perfect sync; their manzai timing is spot on.
チーム全員の息を合わせてプレーしよう。
Let's play with everyone in the team in harmony.
長年連れ添った夫婦は言葉がなくても息が合うことが多い。
Spouses who have been together for years often have a natural rapport without words.
Steam or vapor rising from hot food or drink (a more literary/older use, similar to 'vapor').
鍋から白い息がもうもうと立ち上った。
White steam rose thickly from the pot.
寒い日、熱いお茶の上に小さな息が立った。
On a cold day, a small puff of steam rose from the hot tea.