This page links to several useful dictionaries and other reference material for Mathematics students and teachers.
For definitions, try Jenny Eather’s dictionary
Note too, 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).
Check mathematical formulae from Loughborough University. This is also available as a free app for mobile phones.
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.
The definitions in this dictionary are clear and attractively presented.