Subject specific reference

Cambridge Dictionary of Statistics, 2nd ed.

Concise Oxford Dictionary of Mathematics. 4th ed.

Dictionary of Statistics (Oxford Dictionary of Statistics) 2nd ed.

Dictionary of Weights Measures and Units

Encyclopedia of Mathematics Wiki" This new wiki is a collaboration between Springer and the European Mathematical Society. This new version of the encyclopedia includes the entire contents of the previous online version, but all entries can now be publicly updated to include the newe"st advancements in mathematics. All entries will be monitored for content accuracy by members of an editorial board selected by the European Mathematical Society."

Handbook of Mathematical Formulas and Integrals by Alan Jeffrey, 3. ed. 2003

Hutchinson Pocket Dictionary of Mathematics

Princeton Companion to Mathematics

Springer e-books - refined search: Mathematics: Reference work

Wolfram Alpha A computational knowledge engine for math and much, much more. A few examples of what you can ask WolframAlpha about in mathematics link 

Wolfram MathWorld