Also a netspeak corruption
of the word 'interlude
', most commonly seen in the frame of reference of IRC RPG
Interludes in IRC RPGs are short sessions1 either taking place without a GM2, or with only a few of the regular players (or both).
GM-sanctioned and attendended ludes are often held if a character or sub-group of the party are off by themselves during gameplay, or in unique circumstances like dream sequences. They are usually important to gameplay and the story as much as regular sessions are, but they don't quite count as regular sessions because of the lack of cast.
GM-less ludes are usually only character interaction, and nothing plot-changing usually happens (excepting radical changes in perception of one character by another due to some revealed secret). The GM may give you permission to have one without his/her presence, especially if the GM deems the subject matter not needing any official game officiation (usually this means the characterss involved are going to be talking, and not interfacing with NPCs in any capacity...)
1'short' is relative. Interludes can often take more time than full sessions.
2but usually with the GM's blessing, or the lude is not official.