bridge

The general usage of the term "bridge" is well understood -- a way allowing connection of two separated places.

In a cyberspace context, usage is a little more specific: here, a bridge is a conduit allowing communication between people using different mutually exclusive walled-garden services or protocols, for example allowing one friend on the IRC to have a chat conversation with another friend on Discord and a third friend on Facebook without any of them needing to be personally logged in to each others' services. Typically it is done via bots, user agents lurking like flies on the wall, recording what is written in one place and rapidly rebroadcasting it with attribution through a connected bot logged in to the other services.

Commonly bridged services include the [IRC], Discord, Slack, Telegram, Twitch, Facebook Messenger, Signal, WhatsApp, iMessage, Mumble, WeChat, etc. Matterbridge is the name of one popular bridging service.