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).
This site includes user guides with suggestions for teachers and students.