apex in zone
サブドメインを含まない
ドメイン名
のこと
Zone Apex (Naked Domain) CDN | J-Stream CDN情報サイト
https://docs.netlify.com/domains-https/custom-domains/
root domain
naked domain
bare domain ともいう
Apex domain (also known as a root, bare, or naked domain): the yourcustomdomain.com part in www.yourcustomdomain.com.