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 can be found on the Reference page. If you are interested in the first known use of mathematical words then try Jeff Miller’s ‘Earliest Known Uses of  Some of the Words of Mathematics‘.