Twitter’s built-in URL-shortening domain is t.co.
Twitter introduced t.co to handle links shared in tweets and to give every URL a consistent, compact form. When a user posts a web address, Twitter commonly converts it into a t.co link, regardless of whether the original address is long or already uses another shortening service.
The system is more than a space-saving tool. Twitter can use t.co links to measure clicks and help identify potentially malicious or dangerous destinations before users open them. The visible link may therefore be shorter than the destination URL, while the browser still redirects to the original website.
TinyURL, Bitly, and Ow.ly are separate URL-shortening services, not Twitter’s native domain. Another common mix-up is assuming that t.co permanently replaces the destination address; it generally acts as an intermediate redirect managed by Twitter.