When you come across a word you don't know. Simply select the word, right click and click Look Up in Dictionary. As soon as you do that the dictionary will pop-up showing you the definition of the word.
Additionally, you can open dictionary (its in your Application folder) click Dictionary in the menubar, and click Preferences... and and beside contextual menu select 'Open Dictionary Panel' and you won't even have to open the dictionary and a panel like below will appear.
This tip is especially good when you are reading articles that are full of so many uncommon words that its hard to decipher. In a way, having applications like the OS X dictionary make the computer a much better choice for students and life long learners.
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