The nameserver, or group of nameservers, which has final control over the DNS records which describe a given domain or zone. In order to resolve an URL to an IP address, you must find the nameserver which has the authority for the bottom level domain in that URL.
For example, www.freebsdwiki.net is an URL referencing a third level domain - the first level is "net", the second level is "freebsdwiki.net" - which can formally be described as "the freebsdwiki subdomain of the net domain" - and the third level is "www.freebsdwiki.net", the www subdomain of freebsdwiki.net.
Any given level of subdomain may or may not resolve directly to one or more IP addresses (by use of A records), provide DNS services (NS records), or handle email (MX records). In theory, http://com could be a perfectly valid URL - but in practice, A records are only provided for second level domains and lower.