A distinctionary is an organized and cross-referenced set of distinctions.

As I understand the term, it focuses on providing a range of perspectives to what a term means and doesn't mean, rather than a single categorical definition. In this way, the distinctionary enables greater understanding than might occur through development and use of a dictionary.

One of the most important distinctions is "distinction" itself. A "distinction" is a class of concepts, that opens a new possibility for action. Example, if one knows the meaning of the "purpose" distinction, then s/he can act on it.  

-- PeterMurchland

An excellent way to build such things is the Kellian Repertory Grid, a beautifully automated implementation of which is available for use online at 
http://tiger.cpsc.ucalgary.ca/WebGrid/WebGrid.html (BrokenLink)
http://tiger.cpsc.ucalgary.ca:1500/ (AlternateLink)
Highly recommended. --PeterMerel

Yes, I'm a tremendous fan of the repertory grid enough. For instance, it makes an excellent introduction to a training course to have the attendees spend an hour building a grid around some concepts that the course will address. Much more illuminating that just going around asking people what (they think) they want to get out of the course.  I did this for a testing course I gave last year, and looking back I suspect that the attendees got more value out of having a grid that told them each what they really think about testing than they did out of the testing techniques they were taught afterwards. -- KeithBraithwaite

Very nice idea. But this begs the question of what the distinctions of the distinctions are (and so on). VaguesDependingOnVagues.