"Marriage"

When:

Lookups spiked on June 25, 2011.

Why:

New York became the sixth state to legalize same-sex marriage.

Marriage has become a hot button definition, although its original sense – "the state of being united to a person of the opposite sex" – has not changed. However, because the word is used in phrases such as "same-sex marriage" and "gay marriage" (by proponents and opponents alike), a second definition was added to the dictionary to provide an accurate picture of the word's current use.

Here's more information about how words get into the dictionary.