I love the "define:" tool on Google because:
set googleword to display dialog "What word do you want to define?" ¬
default answer " "
set findgoogleword to (text returned of the googleword)
do shell script "open http://www.google.com/search?q=define:" ¬
& findgoogleword
If you don't want to use Google, then all you have to do is change the address and modify the URL bit to match your preferred site's syntax.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050212092142555