This Diigo list has several useful dictionaries and other useful reference material for Mathematics students and teachers. See Jenny Eather’s dictionary for example and note the section on charts. Also very useful is this glossary for teachers. It’s really good for students too as it defines vocabulary used in Mathematics from Key Stage 1 to Key Stage 4 (Ages 6-16). For the terminology used in question papers see this document: maths exam terminology (I have seen this document around the web, I believe the original is from AQA).
Check mathematical formulae from Loughborough University. This is also available as a free app for mobile phones.
Try Wolfram MathWorld or The Mathematical Atlas. Note that that is possible to obtain a definition from WolframAlpha – for example try complex number for various examples with complex numbers and note that you can choose to look at the mathematical definition also.
For definitions you could also try Jenny Eather’s dictionary mentioned above
or perhaps Or perhaps try the Mathisfun dictionary.