To anyone studying vocabulary, it should hardly be news that Latin has strongly influenced English. But there are also Latin phrases that pepper more academic writing. These phrases are sometimes italicized, and must be treated as inviolable whole, something you cannot just take apart. Meaning if you drop the ‘grata’ from ‘persona non grata’, you get the meaningless ‘persona non’. By extension, ‘grata’ is not a word.
These Latin phrases are not too common on the GRE, but it does not hurt to learn some of the more common ones.
If something is de facto, it is in name only, not necessarily by right. A good example would be a de facto ruler of a country. That person was not elected (he/she is not there by right), however due to some events, that person is the ruler of the country.
Persona non grata
A person who is not welcome is a persona non grata. ‘Grata’ comes from the Latin for pleasing.
Archibald was a persona non grata at family reunions, showing up in garish suits under which he would not so subtly hide his flask of brandy.
If something is required by etiquette and popular fashion then it is de rigueur. It is de rigueur for runway models to be rail thin, hip hop artists to flash considerable bling, and big-name athletes to have an entourage.
Sine qua non
This one definitely looks intimidating, but all it means is an essential something. For instance, in prepping for the GRE, ETS materials are the sine qua non. They are truly indispensible. I like to think of Magoosh as the quasi sine qua non. (Quasi-, by the way, is a Latin root meaning almost).
This phrase actually isn’t as common as the others. It means emerging from nothing. Few works of genius have been truly ex nihilo. Even Isaac Newton himself said he stood upon the shoulders of giants.
This translates literally from Latin as ‘of its own kind.’ A simple way to translate sui generis is unique, or one of a kind.