Encyclopedia of Mathematics

"The Encyclopedia of Mathematics wiki is an open access resource designed specifically for the mathematics community. The original articles are from the online Encyclopaedia of Mathematics, published by Kluwer Academic Publishers in 2002. With more than 8,000 entries, illuminating nearly 50,000 notions in mathematics, the Encyclopaedia of Mathematics was the most up-to-date graduate-level reference work in the field of mathematics.
Springer, in cooperation with the European Mathematical Society, has made the content of this Encyclopedia freely open to the public. It is hoped that the mathematics community will find it useful and will be motivated to update those topics that fall within their own expertise or add new topics enabling the wiki to become yet again the most comprehensive and up-to-date online mathematics reference work"
http://www.encyclopediaofmath.org/index.php/Main_Page