Where does Google pull the results for the define operator

Question

Google has a define: operator which provides a list of definitions for a given word. Where does Google pull those definitions from? i.e. what does Google look for on a page to determine that that page is a definition for a word?

(This has been discussed a number of times on Webmaster World but nobody has come up with an answer yet.)

Answer 1

Last August, they switched to the Oxford Pocket Dictionary.

Seems odd nobody figured this out or at least got close.
Just use the define operator on a few words, eg culture, love, robot.
Then do exact phrase searches for the definitions: culture, love, robot

In each case, the top result is at http://oxforddictionaries.com/ sometimes with slight variation since they probably use a different specific source dictionary of their own.