You can highlight a word in the text of an enhanced e-book (but not a standard e-book) and look up the definition of the word online.

To look up the definition of a word in an enhanced e-book, simply follow these easy steps:

1. From the Tool menu select Select & Zoom/Select Tool.
2. Using the Select tool highlight the word you would like to look up.
3. Right-click (Windows) or command-click (Mac) on the word you have highlighted.
4. From the options list select Look Up.
5. This will automatically launch your default web browser and take you to the word's definition page on Dictionary.com.