Among Cortana’s many skills is her excellent vocabulary, which is powered in the background by the Oxford Dictionaries.
If Cortana suspects you’re trying to look up the meaning of a word, she immediately turns to the dictionary. So if you type (or say) What does disingenuous mean (with or without a question mark), you’ll get a definition of disingenuous. Same goes for parsimony and hegemony and other similarly high-falutin’ words.
But you can force Cortana to look up a word (and save yourself some typing) just by using the right keyword. Click in the search box and type define followed by a space and the word you want.
The top result should be a quick definition, which might be all you need. For more details, click that quick definition or press Enter. That opens an expanded definition, and if that’s not enough you can click to see the full definition on the web.
Cortana has a sense of humor, by the way. If you don’t believe me, just try typing define life.