WolframAlpha can be used for vocabulary, for example try typing ‘plus’ or ‘complex number‘ 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!