百合 is a very common term, and virtually every anime/manga fan knows its association with female-female relationships. It's hard to imagine an otaku who does not understand the meaning of YuruYuri, for example.
However, you have to understand the following points (I explained some of them to you before):
Outside the otaku culture, indeed it's not mainstream at all. Today, 百合 mainly refers to female-female romance in fiction. Wikipedia defines this as follows.
Yuri (百合, "lily"), (snip) is a Japanese jargon term for content and a genre involving lesbian relationships or female homoeroticism in light novels, manga, anime, video games and related Japanese media.
If you want to know words for lesbian used outside otaku contexts, see this question.
百合 is the name of a very common flower, too. Even in the otaku culture, it can simply refer to a flower with absolutely no sexual implication. As I explained before, "banana" has a sexual connotation even in English, but that does not mean it is used in a sexual sense whenever you see it in a novel.
As an aside, if you know the term yaoi, this word fell out of use long ago in Japanese. Young Japanese otaku may not even know this word. (Today, 腐 (ふ; slangy) and BL (sounds relatively "formal") are the common terms.)