zone apex

The bare domain itself, example.com with nothing in front — the name the zone is named after. It cannot be a CNAME. The reason is structural rather than a provider restriction: the apex necessarily carries the zone's NS and SOA records, and a CNAME forbids any other type coexisting on the same name. So the apex takes an A, and an AAAA if the server has IPv6, while www.example.com is free to be a CNAME pointing back at it. Providers offering an "ALIAS" or "ANAME" there resolve the target on their side and serve the address as an A: that is a feature of their nameservers, not a DNS record type. So it disappears if you move the zone, and that is how a migration loses the apex record.