GMCP
Generic MUD Communication Protocol — an invisible side channel next to the text where the server ships game state as JSON.
It began life as ATCP2 at Iron Realms Entertainment, the company behind Achaea, and picked up its neutral name once other games adopted it. It rides on Telnet subnegotiation under option 201. Each message is a package name followed by JSON, for example Char.Vitals {"hp":412,"maxhp":500}.
The point is that the client no longer has to scrape the screen. Players used to write regular expressions against the prompt just to read their own hit points; with GMCP the data arrives ready-made, and the client can draw gauges, maps and lists. It is the same separation of content and presentation the web went through, two decades late and over Telnet.