Lookups on Merriam-Webster.com spiked on November 16, 2010.
After Prince William and Catherine Middleton announced their engagement, the bride-to-be was often referred to as a "commoner."
To many Americans the word sounds comical, but it makes a technical distinction: it describes someone, like Ms. Middleton, who is not of royal or noble birth. For example, as one British newspaper reported,
"Kate Middleton will be the first commoner to marry an heir presumptive to the throne in more than 350 years..." Stephen Bates, The Guardian, November 16, 2010
Commoner has been used with this meaning since the 1300s.
Coincidentally, the sample use that appears in the Merriam-Webster dictionary suits the moment well: "a prince who married a commoner."