in an instant; immediately; right away
Describes something happening very quickly, often with a sense of 'before you know it'. Common in both spoken and written Japanese.
彼は質問を聞くとたちまち答えた。
He answered the question immediately after hearing it.
As soon as it started raining, the street was instantly flooded.
Emphasizes an abrupt, unexpected change. Often used when a situation transforms dramatically in a short time.
The sky suddenly grew dark and thunder began to rumble.
This bread sold out in no time.
The news spread through the whole town in an instant.
He suddenly became a celebrity.