Wiki is one of 287 new words to appear in the online Oxford English Dictionary:

http://news.yahoo.com/s/nm/20070315/wr_nm/britain_dictionary_wiki_dc_2

Says Graeme Diamond, Principal Editor, New Words, OED:

This joins a small but distinguished group of words which are directly or ultimately borrowings into English from Hawaiian. It has been suggested that in some ways the OED itself resembles a wiki: its long tradition of working on collaborative principles means it has welcomed the contribution of information and quotation evidence from the public for over 150 years.