The Kindle has very good dictionary functions and is possibly the all around best. Kindle dictionaries are user changeable and there are a number available, both free and commercial. Also, the Kindle's "back" button makes returning to your reading a simple one button press operation.
On the Kindle word lookup is done as follows:
- use the direction buttons on the 5-way controller to move the cursor to the beginning of the desired word.
- wait 1-3 seconds for the lookup and display of the first short section of the dictionary listing
- either press "back" to clear to dictionary window (if the short section provided the desired info) or press the "enter" key to jump to the full dictionary page.
- page through the dictionary as desired.
- press "back" when done to return to your place in the book you are reading.