WolframAlpha can be used for vocabulary, for example try typing ‘plus’ into WolframAlpha.
Note that WolframAlpha has assumed plus is a character but you can choose to refer to a mathematical definition or a word (or an internet architecture topic). Choosing word will not only give definitions but a wealth of other information that you would not find in a dictionary such as the first known use of the word, frequency of use, rhyming words and much more!
Other sites of interest for Mathematics vocabulary have been mentioned in this Mathematics Reference post. If you are interested in the first know use of mathematical words then try Jeff Miller’s ‘Earliest Known Uses of Some of the Words of Mathematics‘.
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).
and Math is Fun.
There are many resources suitable for older students also, see for example the thesaurus from Cambridge University.
This site includes user guides with suggestions for teachers and students.
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.
LiveBinders version of Mathematics reference materials.