Shoujo-ai

In English, shoujo-ai is a genre of lesbian fiction. Any manga or anime focused on the romantic relationship of a girl with another girl falls in this genre. In Japanese, the word means "pedophilia" instead.

Its gay counterpart is shounen-ai.

Koito Yuu 小糸侑, Nanami Touko 七海燈子, a couple from a yuri 百合 anime.

Left: Koito Yuu 小糸侑
Right: Nanami Touko 七海燈子
Anime: Yagate Kimi ni Naru やがて君になる (Episode 1, Stitch, Censored)

Genre

The shoujo-ai genre is defined by having a focus on lesbian romance, and that's pretty much all there's to it.

If the focus is on girls kissing, flirting, and obscenely hand-holding, that's shoujo-ai. It doesn't matter if it's a drama or a CGDCT anime.

Just having a lesbian couple in a series doesn't make it shoujo-ai.

vs. Yuri

In English, the difference between shoujo-ai and yuri is that yuri is a pornographic genre, while shoujo-ai is not.

If it's lesbian hentai 変態, it's yuri, if it's not, it's shoujo-ai.

In Japanese, yuri 百合 means "lily," the flower, and refers to lesbian romance in fiction in general.

An example of "lilies," yuri 百合, used as a visual pun for lesbian romance.

Left: Kirishima Shouko 霧島翔子
Right: Himeji Mizuki 姫路瑞希
Anime: Baka to Test to Shoukanjuu バカとテストと召喚獣 (Episode 2, Stitch, Censored)

Context: a bunch of lily flowers covering the view.

In Japanese

In Japanese, shoujo-ai means something very different from what it means in English.

shoujo
少女
Young girl. Underage or unmarried.
ai
愛
Love.
shoujo-ai
少女愛
Girl love. Love toward young girls. Pedophilia.
shoujo-ai-sha
少女愛者
Someone with shoujo-ai. A girl lover. A pedophile.
- Also known as a lolicon ロリコン.

The word has nothing to do with lesbians at all!

How did this happen?

When the west was importing genres from Japan, before 1980, there was a gay genre called shounen-ai 少年愛, meaning literally "boy love," which was found in shoujo manga, that is, in manga whose target audience are young girls.^{(ja.wikipedia.org)}

Sunroom ni te サンルームにて, published in 1970 on Bessatsu Shoujo Comic 別冊少女コミック, is said to be the first shoujo manga featuring a gay kiss.

Although this shounen-ai genre existed, there was no shoujo-ai, "girl love," genre.

There was a lesbian genre called "S," esu エス (katakanized alphabet letter), from the word "sister."^{(ja.wikipedia.org)}

Presumably, someone took one term, replaced one word by another, and that's how shoujo-ai was born.

It's a Japanese word made in English, an eisei-wago 英製和語.

Ironically, nowadays gay fiction is called BL in Japanese, which is short for "Boys' Love," while lesbian fiction is called GL, which is short for "Girls' Love."