Updated June 5th, 2019.
Macs come with a Dictionary app. It’s very handy. Find it in the Applications folder.
When you launch it, you get this (you will, that is, if you type “sleep” into the search box):
EVERY word in the Dictionary is clickable! I mean “every”– and that includes words in the definitions! So try clicking. Also, try the thesaurus and the Apple and Wikipedia sections.