adverb
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.
雨が降り出すと、通りはたちまち水浸しになった。
As soon as it started raining, the street was instantly flooded.
adverb
Emphasizes an abrupt, unexpected change. Often used when a situation transforms dramatically in a short time.
空がたちまち暗くなり、雷が鳴り始めた。
The sky suddenly grew dark and thunder began to rumble.