is an adverb in that is used to convey the feeling of something happening 'suddenly', or 'in a flash'. The primary nuance with たちまち
compared to other similar structures is that たちまち
indicates that something happened spontaneously, and was not able to be controlled.
In many cases, たちまち
will indicate something entering a new, ongoing state, or something being completed.たちまち
will often appear directly before the verb phrase that it is describing as being the new state.
Artists who belong to that record company quickly become famous.
Tanaka-san and I immediately became friends.
The cinnamon rolls I had put in the fridge quickly disappeared into my wife's stomach.
Fun-fact - たちまち originally comes from the kanji structure 忽（たちま）ち, with the kanji itself meaning 'without thought'. This nuance is kept with たちまち, as all actions associated with the word indicate a lack of planning or thought before happening. Despite this, the kanji itself is rarely used in modern writing.