Why I can't use the 'define' button after updating to iOS 9.0.2?
Go to Settings -> General -> Language &amp; Region -> and make sure the languages you want your iPhone to recognize are in the "Preferred Language Order" list.

If the language you prefer is already in that list, tap "edit" to delete the language, and then add it back up. This should add the language's dictionary and enable the "define" function.