Cortana will now help you define words
Cortana can now provide definitions
We should point out that if you are an Android or iOS user, you might not be terribly impressed since both Siri and Google Now both already handle definitions. By the way, in case you were wondering, Cortana gets the definitions she repeats from the Oxford Dictionary, so you should feel confident with her responses.
While Cortana should be able to define any real word, there are some Windows Phone 8.1 users who have Cortana, but not the capability to handle definitions yet. Do you know what the word disappointed means?
source: Neowin via WMPoweruser