TTL

The duration in seconds a DNS answer may be cached by a resolver before being discarded.

In plain words

TTL is the lifetime of a DNS record in cache. Until it expires, cached resolvers keep serving the old value.

Practical example

A TTL of 3600 means a DNS change may only become visible one hour after it is published.

Where you encounter it

When changing DNS records and waiting for the changes to propagate across the internet.