I have an expansive view of what constitutes an outliner - the ability to manipulate a hierarchy. Even expanding and collapsing might not be included (I feel like BrainStorm on Windows is a type of outliner even though you don't expand or collapse nodes, you simply zoom in and out of them). On the flip side, I think I know what people mean when they are talking about a dedicated outliner - but I could be wrong. The distinctions single-pane, two-pane, etc., seem pretty well understood and useful.

However, even my definition confuses me because I consider a mind mapper something related but not quite equivalent to an outliner, even though it technically fits my definition.

In any case, I do love that there is a place where heated debate can ensue over the definition of an outliner!