What is the motivation behind naming the category O appearing in the theory of Lie algebras? Does O stand for something? 
Here is a question [Why the BGG category O?](https://mathoverflow.net/questions/64931/why-the-bgg-category-o) that further confuses me. It seems like there is a notion of when a category is O, is it?



Why the name O for category O?
From [Humphreys: Representations of semisimple Lie algebras in the BGG category O], notes for Chapter 1:



> 
> The letter chosen to label the category is the first letter of a
>  Russian word meaning “basic”
> 
> 
> 


which is основной.


