I cannot present this material in the manner which I desire. LiveJournal scrubs links, removing important structure. So, instead of one instruction, here are eight:
If you are a Firefox user, right-click on the area below the location bar but above the row of tab labels.
Select "New Bookmark...".
Name the bookmark "Look It Up".
Set the location of the bookmark to be
Visit some web page which contains a word the definition of which you would like to know.
Select the word in the document (for example, double click on it).
Click the "Look It Up" bookmarklet you just added.
Savor the novelty (it wears off fast, so savor it likewise).
Maybe this is convenient for something. However, it does demonstrate document range selections, which have some other interesting uses. For example, I would like someone to write a Mantissa application which tracks quotes this way. People should be able to vote for quotes they find on the web. The quote can easily be selected and submitted to the server for tracking. The server can keep running tallies for various time periods and report them. It can also do things like indicate the first reporter of a quote, long-persisting quotes, etc. Communities like comp.lang.python should find this useful, as people are always responding to posts with messages that consist of nothing more than "+1 QOTW".
This would be an extremely trivial Mantissa application. Who would like to write it?
Props to radix for reminding me that dict.org is way better than dictionary.com.