The default monolingual American English Dictionary for the native speakers of English is pretty good. You can look for some examples yourself by going to the website version
http://www.oxforddictionaries.com/us and type a word or phrase.
The Kindle also comes with a similar dictionary for British English.
If English is not your first language, you can buy additional dictionary such as Merriam Webster Advanced Learner Dictionary for the Kindle
https://www.amazon.com/dp/B001GX3ZWA
It was quite good but I rarely use it now since my English has improved and I now prefer using the one for native speakers instead.