The Diigo Reference list (see I’m Looking For…page for this and other lists) includes many useful resources including Mathematics dictionaries.
Note – click on the green links to go direct to each site then back on your browser if you wish to return to the list.
Note that these resources are also given on my blog written for students.
Two particularly useful dictionaries for students of all ages are those from Jenny Eather
(Note the excellent section on charts also).
The Diigo Reference list also includes sites on the history of Mathematics.
Checking the University of St Andrews site we can find out about mathematicians who were born or died on this day.