realtime chat
About
Realtime chat (RTC) software is designed to allow users to interact with little or no perceptible delay between one person's post and others being able to see it. There is also almost always some indication when other users are typing.
The ancestor of all realtime chat software is arguably IRC.
Proprietary
The most popular RTC services are Discord and Slack, both proprietary and available via web, desktop, and mobile app: