Instantly means happening immediately, with no delay at all. When you flip a light switch and the room brightens instantly, there's no waiting, not even for a second. When you drop a glass and it shatters instantly on the floor, the breaking happens the moment it hits.

The word describes things that occur so fast they seem to happen in zero time. A calculator solves math problems instantly. When you recognize your best friend's voice on the phone, you know instantly who's calling. A chameleon's tongue can catch an insect instantly.

Instantly suggests pure speed without any pause or hesitation. Compare it to quickly, which just means fast: a quick response might take a few seconds, but an instant response happens right now, in this very moment. When something happens instantly, you don't have time to blink or think.

People sometimes exaggerate and say things happen instantly when they really just mean very fast, like claiming they instantly became friends with someone when it actually took a few days to get to know each other better.