A built-in dictionary and the Dictionary application can be used to look up definitions and words similar to those in a text.
This feature is supported in Safari as well as other apps following the same instructions.
For more information: https://support.apple.com/kb/PH21573
1. Trackpad Look Up – Dictionary
Looking up a word in a built-in dictionary and thesaurus is achieved by lightly tapping three fingers on the trackpad over the word.
2. Right-Click Look Up
You can also look up words in the built-in dictionary and thesaurus by right-clicking on the word in an article. Then select Look Up.
3. Dictionary App
The Dictionary application can provide dictionary and thesaurus references for multiple languages. Find the Dictionary application in your Applications Folder.