The word 'imperative'

When I type the word ‘Imperative’ or ‘imperative’ in a window, save the window and close it, when I reopen the window, the word is highlighted (see attached image). I have searched in vain through the preferences for something like ‘keywords’ – words that are automatically highlighted for example.

In the process of trying to get to the bottom of this I discovered that the “System Text Preferences…” button in Preferences>Corrections appears to do nothing. But then, I note that selecting “Language and Region” causes the Mac OS preferences to freeze. Such a treat.

I am on a Mac using OS 10.13

Thanks for any help
Screenshot 2017-10-12 10.18.00.png