1 [usually singular]
She caught his eye for an instant.
When the rain started, the crowd vanished in an instant (=immediately).
an instant of panic
She stepped towards the door and in that very same instant, the doorbell rang.
as soon as something happens:
The instant I saw him, I knew he was the man from the restaurant.
Jen burst out laughing the instant she walked in.
used when telling someone, especially a child, to do something immediately [= now]:
Come here this instant!