A domain with a name containing one "dot": for example, "ipr-helpdesk.org", "google.com" or "lemonde.fr". This can be distinguished from a third-level domain, containing two dots (such as "qmul.ac.uk"), a fourth-level domain, containing three dots (such as "seda.demon.co.uk"), etc.(ES:Dominio de segundo nivel, IT:Dominio di secondo livello, FR:Domaine de second niveau)
The domain name system is designed as a hierarchy. The root is the highest level of the hierarchy, followed by the top level domain, then followed by the second-level domain. For example, for the domain "espn.com", "espn" is the second-level domain, "com" is the top level domain, and the "." is the root.
a child of a first-level domain, and so on