without delay or hesitation; with no time intervening
"he answered immediately"
"found an answer straightaway"
"an official accused of dishonesty should be suspended forthwith"
"Come here now!"
At once; without delay.
"The lights flipped off instantly as soon as I stepped out of the room."
In plain English: Instantly means something happens right away without any waiting time at all.
"The door opened instantly when she pressed the button."
Usage: Use instantly to describe an action that happens immediately or with zero delay between a cause and its effect. Avoid using it to mean something that occurs quickly but still takes a measurable amount of time.
as soon as, directly.
"The lights flickered and instantly went out."
The word instantly comes from combining the adjective instant with the suffix -ly to form an adverb. It entered English directly as a modification of the existing noun and adjective meaning "occurring without delay."