When Twitter started, its most obvious characteristic was its brevity. Users were limited to sending messages no longer than 140 characters, brief enough to fit into the short message service (SMS) system, the standard text-messaging system used by mobile phones around the world.

But for business customers — developers looking to integrate with Twitter — the length constraint for messages was secondary. The most interesting thing about Twitter was its API (or rather, that Twitter was an API).

That’s one of those computer-industry acronyms that doesn’t really stand for anything. It originally stood for “application programming interface” and later “advanced programming interface,” but really “API” just stands for API — a subtle, sui generis way of talking about a feature of a digital system that has no adequate equivalent in the nondigital world.