Role Playing Game...
In the past, games were much easier to categorize into genres, like music in the past, but... Like music... The industry has evolved and games can incorporate many different aspects of genres and be harder to easily define.
At first, there were (and still are) subsets of genres, like "Action RPG", "Turn Based RPG, JRPG, Open world, blah blah blah...
Games can have RPG elements and still not be a full fledged RPG. Those elements being...
- Taking the role of a character set on an adventure
- Character Progression (Levels, abilities)
- An epic story
- Interaction with NPCs
- Various locales to explore (dungeons, towns, etc.)
- Menu based management of characters, items, maps, etc.
And more...
Edit:. I agree with the poster who stated the fact that the term came from tabletop role playing games that had a lot of these elements in analog form.