EMZETT.Second-Level Domain
Image: Unbekannt, CC BY-SA 2.5, Wikimedia Commons
In short: The freely choosable part of the name directly before the TLD — for emzett-digital.com, that’s emzett-digital.
In more detail: The second-level domain is registered with a registry (registrar) and is the part you typically “buy”. Any number of subdomains can be created underneath it.
In Depth
When registering a second-level domain, the registrar first checks whether the desired name is still free under the chosen TLD — if it’s taken, the only options are buying it from the current owner (often at a considerably higher price than a new registration) or choosing a different name or a different TLD. Registration usually happens one or more years in advance and has to be renewed afterwards — if it expires, the domain is released again after a grace period and can be newly registered by anyone.
Technically, the second-level domain is the level where most economically relevant decisions are made: it carries the actual brand name (emzett-digital), while the TLD and subdomains have more of a structural/organisational function. Some countries with ccTLD structures also have a fixed “second-level” category below the TLD (e.g. .co.uk, .com.au), where the actually registerable level sits one step deeper than with simple TLDs like .com.