Wiki
A wiki is a web site designed to be extremely easy to edit.
The original wiki is at http://c2.com/cgi/wiki. Nowadays there are many more wikis. Wikipedia is only one example of a wiki, and not a very typical one, but it’s much better known than the others, so some people think “wiki” is short for “Wikipedia”, which is annoying … I’ll all in favour of Neologisms, but not when they try to overwrite really useful preexisting words!
See http://en.wikipedia.org/wiki/Wiki for more definitions.
See http://c2.com/cgi/wiki?WikiDesignPrinciples for Ward Cunningham’s original ideas about what a wiki should be like. Bolzano breaks the openness principle by requiring a password for editing (because without the immense user base of Wikipedia it’s hard to deal with spam) but follows all the other principles.