The Domain Name System maps hostnames to Internet address using a hierarchical namespace where each level in the hierarchy contributes one component to the FQDN. For example, the computer foldoc.doc.ic.ac.uk is in the doc.ic.ac.uk domain, which is in the ic.ac.uk domain, which is in the ac.uk domain, which is in the uk top-level domain.
A domain name can contain up to 67 characters including the dots that separate components. These can be letters, numbers and hyphens.
See domain theory.
5. <programming> A specific phase of the software life cycle in which a developer works. Domains define developers' and users' areas of responsibility and the scope of possible relationships between products.
6. The subject or market in which a piece of software is designed to work.