This page defines the term. For wording you can copy, see the WhatsApp away message templates.
An away message is set in the WhatsApp Business app with one of three schedules: always send, custom schedule, or outside of business hours (available once business hours are filled in on the profile). Recipients can be everyone, everyone not in the address book, everyone except chosen contacts, or only chosen contacts.
It complements the greeting message, which welcomes first-time contacts. On the API side, the same job is done by automation rules and off-hours flows, which can also route urgent requests differently.