A Channel (in a Market Data context) is a multicast group and port tuple. VMX-Capture probes are configured to listen to channels to get feed data.
In generic terms, this is a stream. Different venues may split channels up in different ways, for example, by ticker or by Matching Engine etc.
In Beeks Analytics feed definitions, a Channel sits in the hierarchy as follows:
Venues
Markets
Feeds
Channels
Side
Beeks Analytics for Markets aggregates by Feeds, Channels and Side.