If something happens in an instant, it happens very fast or it happens immediately.
Microwaveable food is ready in an instant.
There was an instant response by the police who arrived immediately after gun shots were fired.
The medication prescribed by the doctor provided instant relief.
Polaroid instant print cameras provided instant photographs that popped out from the camera itself.
You can buy instant rice at the grocery store, but why not make regular rice? It’s already easy to make and it tastes better than instant rice.
Instant mashed potatoes are also available at the grocery store, but why would you buy a box of instant mashed potatoes?
Many Americans expect instant gratification for any sort of want or need. Instant gratification is part of the culture. (This means that if you want or need something, you should be able to get it immediately.)
Coffee makers can make coffee in an instant. (The phrase, in an instant, is commonly used to mean that something will be available quickly.)
I’ll have that for you in an instant.
Fast food places have your order ready in an instant.
The word "instantly" is an adverb:
The pain relievers worked instantly.
Meals were instantly prepared as soon as they were needed.
ATM machines can instantly provide cash to people who have bank accounts.