Suggestions for mathematics encyclopedia

The suggesstion by Peter Humphries in the comments is good. This book contains a nice overview of many fields in mathematics, although you might find that it does not contain the level of detail you're looking for.

There is also Springer's online Encyclopaedia of Mathematics at http://eom.springer.de that might be what you're looking for. Personally though, I usually do a google search and end up at Wikipedia, or look into a reference book of a specific field, for example Blackadar's "Operator Algebras".

The Encyclopedic Dictionary of Mathematics (http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=7771) sounds closest, at the level of MO. It is two volumes, and is of Japanese origin. The Springer Encyclopaedia of Mathematics, all online now at http://eom.springer.de/, is an updated version of a Soviet work. Neither of these, however, is going to be a good reference for discrete mathematics. I'm not aware of a book that does what you ask.

Since you mention Discrete Mathematics the 'Handbook of Combinatorics' (in two volumes, total of 2000+ pages) could be interesting to you. Roughly, it is an organized collection of survey articles covering major fields and techniques of combinatorics (few or no proofs, but lots of definitions, results, and references), each written by an expert in the respective subfield.

I think it is a very nice book; yet, at least a couple of years ago, it was (in my opinion) a bit expensive.

There is another book that seems similar 'Handbook of discrete and combinatorial mathematics' (1000+ pages), which however I do not know.

The more precsise bibliographic informations are:

Handbook of combinatorics. Vol. 1, 2. Edited by R. L. Graham, M. Grötschel and L. Lovász. Elsevier.

Handbook of discrete and combinatorial mathematics. Edited by Kenneth H. Rosen, John G. Michaels, Jonathan L. Gross, Jerrold W. Grossman and Douglas R. Shier. CRC Press.

There are similar handbooks for other fields, too.