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Icecast is free server software for streaming multimedia.

Icecast 2.0.1 Docs — Glossary

Please note: This doc is for an older Icecast version, you can find the latest doc here.
Source client
A source client is an external program which is responsible for sending content data to Icecast.
Some source clients that support Icecast 2 are Oddcast, Ices 2, Ices 0.3 and DarkIce.
Slave server (Relay)
The slave server in a relay configuration is the server that is pulling the data from the master server. It acts as a listening client to the master server.
Master server (Relay)
The master server in a relay configuration is the server that has the stream that is being relayed.
A mountpoint is a resource on the Icecast server that represents a single broadcast stream. Mountpoints are named similar to files (/mystream.ogg, /mymp3stream).
When listeners connect to Icecast, they must specify the mountpoint in the request (i.e. Additionally, source clients must specify a mountpoint when they connect as well. Statistics are kept track of by mountpoint. Mountpoints are a fundamental aspect of Icecast 2 and how it is organized.
Fallback mountpoint
A fallback mountpoint is configured with a parent mountpoint. In the event of the parent mountpoint losing connection with Icecast, Icecast will then move all clients currently connected to the now defunct mountpoint to it’s fallback mountpoint.