This Diigo list (click on the green links to go direct to the site and then back on your browser to return to the 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).
Some students may have seen 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. Apps for GCSE (ages 14-16) and A Level are available.
For older students perhaps 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.
Or perhaps try the Mathisfun dictionary.
Note that Diigo is a social bookmarking tool which allows you to store your favourite websites online and share them with others. This means you can access your favourites from any computer.