A text file that contains DNS records for a domain. It maps domain names to IP addresses, mail servers, and other resource records. Your registrar or DNS host maintains this; you edit it to point traffic where you want it to go.
Zone files use standard record types: A (IPv4), AAAA (IPv6), MX (mail), CNAME (alias), TXT (text/verification), NS (nameserver), CAA (certificate authority), SRV (service), and others. Each record has a TTL (time-to-live) that tells caches how long to keep it.
Why it matters: If your registrar goes dark, gets raided, or simply ignores you, a zone file in your control (hosted on your own authoritative nameserver or a bulletproof DNS provider) keeps your domain live. Registrar lock or DNS hijack becomes irrelevant if you control the actual records. This is why serious operators run their own nameservers or use providers with no abuse desk and no takedown habit.
Zone transfers (AXFR) between master and slave nameservers can leak your full DNS configuration if misconfigured — don't do that unless you trust the recipient. DNSSEC signs the zone to prevent tampering.