brief, fleeting
For a brief instant, I thought she was going to fall.

given, one, single
His news was too important to be contained for a single instant longer.
At any given instant the distribution of molecular speeds is always constant under the same conditions.

very
He took out his keys to lock the door. At that very instant the door flew open and a man ran into the room.

hesitate for, pause for
He paused for an instant before continuing.

at … instant
The bomb could go off at any instant.

for an instant
Just for an instant I thought he was going to refuse.

in an instant
It was all over in an instant.

the next instant
The next instant she was flying through the air.

instant after, instant before
She woke up in the instant before the phone rang.

be

almost
His response was almost instant.

Instant is used with these nouns:
access, answer, appeal, celebrity, classic, coffee, credibility, cure, death, dislike, dismissal, fame, feedback, hit, impact, insight, liking, message, rapport, recognition, relief, replay, retaliation, smash, solution, success, transformation, wealth