What is a "splinter meeting"? Like this one link.

It seems nobody describes it in detail.
A splinter meeting is a relatively small meeting that takes place at a larger event, such as a conference. It is likely organised by someone besides the conference organisers, and while it's typically related to the topic of the conference it may be much more specific in focus. For example, I've been at psychology conferences where splinter meetings have been held for users, or potential users, of pieces of technology that are used in research.

A similar idea is that of satellite meetings/conferences/workshops which are smaller, more specific events held in association with a much larger, more general event. They take place before or after the larger event (not during, like a splinter meeting might). Often in different locations and usually with different organisers. They are, however, often promoted to some extent by the larger event.