DNS and domains
TTL (time to live)
The number of seconds a DNS record may be cached. It decides how long a change takes to reach everyone.
Every DNS record carries a TTL. A resolver that fetched the record keeps answering from its cache for that many seconds before asking the authoritative server again. 3600 (one hour) is a common default; 86400 (one day) is common for records that rarely change.
The TTL is why a DNS change is not instant. If a record had a TTL of one day, resolvers that fetched it just before you changed it will keep the old answer for up to a day. The usual trick before a planned change is to lower the TTL to 300 a day in advance, make the change, and raise it again afterwards.
Low TTLs cost more queries and give you faster changes; high TTLs mean fewer queries and more resilience when your nameservers have trouble. For records you may need to move quickly, such as the address of a web server, a few minutes to an hour is a reasonable range.