This Diigo list has several useful dictionaries and other useful reference material for Mathematics students. 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). Note the KS1 – KS3 new edition from NCETM. 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).
Try the excellent credit card size information sheet full of mathematical formulae from Loughborough University. This is also available as a free app for your mobile phone.
From New Zealand Maths, check this very comprehensive glossary of mathematical terms.
For older students perhaps try Wolfram MathWorld. 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.
Or perhaps try the Mathisfun dictionary.