In an instant; immediately; happening very quickly—used to say something occurs right away.
たちまち部屋が静かになった。
The room went quiet in an instant.
薬を飲むと痛みがたちまち和らいだ。
When I took the medicine, the pain eased immediately.
知らせを聞くと彼はたちまち駆け出した。
When he heard the news, he ran off at once.
Suddenly; all of a sudden; used when something changes abruptly or happens all at once.
空が曇り、たちまち嵐になった。
The sky clouded over and suddenly it became a storm.
観客は驚いてたちまち静まり返った。
The audience was surprised and suddenly fell silent.
事件の噂が広まり、たちまち街は混乱した。
Rumors of the incident spread and the town was suddenly thrown into confusion.