X-Chat Aqua uses the term "tabs" to refer to the various streams of information it presents -- channels, private chats, notices/snotices, and server messages.
This term is a bit implementation-centric to begin with -- it's like calling a series of options "Checkboxes" instead of "Options". It makes even less sense in Mac OS X 10.3 or later, because the controls X-Chat uses are not tabs, they're multi-part buttons (aka "segmented controls"). And it makes even less sense when "Show tabs at: Outline" is selected, because then an expanding list is used, and the list items don't look even *slightly* like tabs. :-)
There probably is no good name to encompass all the types of information X-Chat presents this way, but "Channels" would make more sense overall than "Tabs" does. This term could be used as the heading of the outline listbox, the name of the Preferences category, and throughout that Preferences pane.