URL

An address with a wider reach than the web alone: the same scheme locates an FTP site, an audio stream or any other Internet resource, though websites are by far its commonest use. It is an ordinary count noun, so the plural is a plain URLs with no apostrophe. Looser uses grow from it, such as the "domain name" sense.

Here the part stands for the whole. This use belongs to the domain trade: cybersquatters buy URLs close to famous ones, typosquatters register misspellings of popular sites, and speculators hold URLs as real estate to resell later. Strictly, a domain name is only one piece of the full web address, so careful technical writing keeps the two apart.

Tumblr slang. A blog's username sits inside its web address, so on Tumblr naming one names the other, and users simply call the username the URL. It is a second step of the same shorthand behind the "domain name" sense, and outside Tumblr it will puzzle readers.

A homograph from American military and nautical usage, unrelated to the web address: here URL abbreviates unrestricted line officer, and URLs names a class of officers. Context usually separates the two at once. In writing where both could appear, such as a naval website or IT manual, spell out the full term on first use.

A term from laboratory medicine and statistics: here the letters abbreviate upper reference limit, the top boundary of a reference range for a measured value. Like the naval initialism, it shares only its spelling with the web address. In a clinical report it pairs naturally with a lower limit, and results above it are flagged.