If you want a free dictionary with English definitions for your computer, Tray Dictionary is a great option.
This application stays in the background in the system tray and only runs if you click on it. When you do, a small query window for the word you want to look up will open automatically.
After you type in the word, the application will show different variants in case there is more than one, each one accompanied by its corresponding definition.
Tray Dictionary uses the prestigious Cambridge Dictionary of American English for its results. This way, its search results are guaranteed to be reliable.