OS X El Capitan: Look up words
You can quickly look up the definition of a word while working in apps and while browsing webpages. If you’re using OS X v10.10.3 or later and Spotlight Suggestions are enabled in Spotlight preferences, you may also see information about related music, movies, or map locations.
Look up a word: Control-click the word or phrase (in some apps you may need to select it first), then choose Look Up from the shortcut menu. Or tap the word with three fingers.
If your trackpad supports it, you can force click the word. (To force click, press firmly until you feel a deeper click.)
The dictionary definitions shown vary, based on the sources set in Dictionary preferences. You can open the Dictionary app from Launchpad in the Dock.