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in an instant; in a moment; immediately; right away; at once
suddenly; all of a sudden; all at once
噂はたちまち小さな町中に広がってしまった。
The rumor spread through the town in no time.
薬を飲んだらたちまち熱が下がった。
After taking the medicine, the fever went down immediately.
彼の作品はたちまち人気になった。
His work became popular in no time.