Mac users – did you know that your Spotlight search feature (the little magnifying glass icon at the far left of the menu bar) gives you quick access to the Dictionary app?
As you type the word you want to look up, the Spotlight panel displays a list of things on your system matching your search. One of those is the Dictionary definition. Click on it and it will take you right to that word in the Dictionary app. Unfortunately, it won’t help you with spelling – you have to spell the word correctly before it will show up in the Spotlight results. The Dictionary does include a Thesaurus in the lookup so this is also a quick way to find just the word you want.