Rich Communication Services
A global industry standard for phone messaging and multimedia communication. It replaces SMS and MMS with a new platform that is more modern and flexible.
Just as VoLTE is a more modern standard for voice calls, RCS is the equivalent update for messaging and media, including chat, video, and file sharing. Both VoLTE and RCS update the industry standards for mobile to be fully IP-based (like the Internet), which is one of the key goals of 4G mobile network technology.
As a replacement for SMS/MMS, RCS includes modern chat features like presence, so you can see when the other party is offline, when they are typing, and whether they received messaged you have sent. The RCS standard also supports functions such as large file sharing and live video sharing.
RCS is intended to be built into phones as part of the default messaging app/experience, and is planned to be as ubiquitous and interoperable as SMS.
RCS is essentially the Android equivalent of Apple's iMessage. Apple is invited to support RCS as well, but as of 2019 they have not.
As of 2019, RCS has been made available on a few phones, but adoption has been slow. Android phones support RCS as part of Google's Messages app, but RCS only works when carriers support it on specific phone models. The top US carriers have launched a joint venture that aims to roll out RCS faster in 2020 and beyond.