Definition: A sort of pseudo-reality that exists when you make something up and enough people agree with you. Coined by Stephen Colbert, reporter for Comedy Central’s Colbert Report.
Source: Can Wikipedia Handle Stephen Colbert’s Truthiness? (MTV News)
Of course, that begs the question of what to call the ever-changing “reality” that is found on Wikipedia, since popular terms may be edited nearly constantly throughout the day and an entry is almost never definitive. Instabiliality?