For the Nook 1st Edition: Tap "Look up word." Tap "Keyboard." Enter the word you want to look up and tap 'Search." Tap "OK" and "Cancel" when you are finished.
The Nook Simple Touch only appears to let you look up words which appear in the text you are reading. to do this, press and hold a word you want to look up, and a menu opens. Select "Look Up."
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