Go to google, or, if you're in Firefox, just click or select the Google search field. Type 'define: something' - something can be anything you're trying to look up.
Google returns with a (usually fairly accurate) guess about the definition of that word, based on other lookups.
Since I've gotten totally annoyed with sites like Dictionary.com and the like (which regularly take 20 seconds just to come up with the input form), this is a great shortcut.
In Firefox you can simply type "dict word" in the URL field and it will take you directly to the dictionary.com site, with the definition you are seeking.
Interesting - I didn't know that - though it seems to go to 'die.net', someplace I hadn't heard of.
What I love even more is to SMS "GOOGL" from my mobile with the message "d: something," and get the definition to my cell phone. I use it as a mobile spell-check when I'm away from a computer. You can also put in "comics 02144" and get back yellow-pages entries for (e.g.) comic book stores, but it's not very complete, yet.