domain

Territory turned into a metaphor: the ruled estate becomes the ground a person's knowledge or authority covers. It sits naturally after a possessive (my domain, his domain) and in phrases of limit such as not really my domain, a polite way of sending someone elsewhere. In research writing it is often plural, naming the separate areas a study measures, such as daily living and social skills.

The sense closest to the word's roots: through Old French domaine from Latin dominium, "right of ownership", and behind that dominus, "master". Demesne is the same word arriving by another road. The ruler need not be human. Trevor Holloway, in Railway Magazine (1954), grants a domain to bears and other ferocious beasts, and Christopher de Bellaigue in The Guardian (2020) measures British farmers' domain as most of the country's surface.

In the Domain Name System the name and the territory are the same thing. The BIND 9 Administrator Reference Manual of 2000 puts it flatly: every name in the DNS tree is a domain. In everyday use, though, people mean a delegated name, such as a company's, which stands for everything registered beneath it.

A function's domain is where it is allowed to work. Argument is the mathematician's word for an input, so "argument values" and "inputs" mean the same thing here. Finding a domain is usually a matter of subtraction: start with every number and remove the ones that break the rule, such as values that make a denominator zero. The relation-based version widens the idea to sets of pairs.

Taxonomy's top rank comes from the three-domain system that Carl Woese introduced in 1990. It set Bacteria, Archaea and Eukaryota above the kingdoms on the basis of differences in ribosomal RNA. The names of individual domains are capitalized as formal taxa, while the rank itself stays lowercase: the domain Archaea.

Proteins are built in modules, and a domain is one module: a stretch of the chain that folds into a stable shape and does one job, often even when cut away from the rest. Biochemists label them by function, as in a catalytic or binding domain, and one protein may carry several. Applied to a chromosome, the word marks a segment that is distinct in the same functional way.

Physics borrows the territorial image for magnetism: a ferromagnetic material is a patchwork of small regions, each with its own magnetization direction. The boundaries between neighbors are called domain walls. A piece of such material can be unmagnetized overall simply because its domains point every which way. A field that pulls them into line magnetizes it.

Where the single name is a label, here the word means the whole family that name heads: the delegated name plus every subdomain under it. The difference matters when a domain is moved or secured, because the operation covers the entire subtree, not one record. "DNS-like" extends the word to naming schemes built on the same tree pattern.

Network administrators speak of computers being on a domain or added to it. Here the word means the machines themselves rather than their names. A server added to the domain becomes one of the computers its names identify. As a result, the same word can describe a namespace in one sentence and a roomful of hardware in the next.