Search and the web
One site linking to you, however many times it does so — the unit that carries information, and not the one quoted in proposals.
Also called Linking root domain · Ссылающийся домен
A thousand links from one site is one site's opinion of you, repeated. Ten links from ten sites is ten opinions. Every engine has treated these differently for two decades, and every link-building proposal quotes the first number because it is the one that can be manufactured cheaply — a sitewide footer link on a single domain produces as many links as that domain has pages.
The count is also not the quantity of interest on its own. A link that no crawler has fetched does not exist as far as the index is concerned; a link marked nofollow, sponsored or ugc is a declaration by the linking site that it is not vouching for you. A referring-domain count that does not separate these is a count of markup.
The pricing unit tells you what will be delivered. A campaign quoted per link is optimised to produce links, which is a different objective from producing sites that link — and the cheapest way to hit it is more links from the domains already used. Ask for referring domains, ask which are indexed, and ask which carry a nofollow; the three questions usually reduce the deliverable by an order of magnitude before anything is bought.
The definitions are the easy part. Whether the figure on your dashboard was computed this way is a different question, and usually the more expensive one.