When talking about lesbian relationships in anime and manga, we often use the word yuri. (When talking about homosexual relationships between men, we use yaoi.) What does it mean originally and how did it come to be used as a synonym for girls’ love?
The word 百合 literally means lily, as in the white flower. I know there are lilies of countless colors (we have some orange ones in our garden), but the 百 in 百合 means white and the original Madonna lily is white.
Now, regarding the second part of the question, how it came to be used as a synonym for girls’ love. In the seventies in Japan, there was a magazine for the gay male community, called 薔薇族. If you’re into yaoi, you might know that there is a genre called 薔薇, which refers to gay manga made (usually) by and for gay men. (To be contrasted to yaoi, which is usually drawn by and for straight women.) Anyway, in 1976, Bungaku Itō, editor of 薔薇族, used the term 百合族 in reference to female readers in the title of a column of letters called 百合族の部屋. The column didn’t last long and appeared just sporadically, but since it was geared towards lesbians, that name stuck. So gay men were the roses, while lesbians became the lilies.
Take this explanation with a grain of salt though. Who knows, maybe this wasn’t the first instance of lilies used as a synonym for girls’ love. Maybe it came from so many main characters in yuri manga being called 百合 or 百合子. (I wonder how that idea would have blossomed without the magazine popularization.) Mr. Itō is still alive (85 years old this past spring), but I doubt he remembers where the name came from.