Rabba = Female Rabbi.

 Jweekly has this interesting article about the rise of the term "Rabba" to describe a female rabbi.

Stern-Kaufman, who was ordained by the pluralistic Academy of Jewish Religion in Riverdale, N.Y., became just the second woman in the United States to be given the title rabba. She followed the lead of Rabba Sara Hurwitz of the Orthodox Hebrew Institute of Riverdale.

...

She did find, though, that it was a hot topic in Israel. She noted that the Academy of Hebrew Language in Israel accepted in 2010 the word rabba as part of the Hebrew language.

“Rabba is an up and coming title in Israel for liberal women rabbis,” Horn Prouser said. “Anyone I talked to in Israel said, ‘Oh yes, I know rabba so and so.’ ” 

Weirdly enough as a religious Jew in Israel, I've yet to come across a single Rabba. I'm fairly sure that if you were to ask the average Hebrew speaker what a female rabbi is called, he would answer "Rabbanit". I had a quick look at the website of the Acadamy of the Hebrew Language showed this explanation:

צורת הנקבה של רַב המשמשת זה דורות היא רַבָּנִית (בדומה לצורת הרבים 'רבנים'), אלא שמילה זו משמעותה 'אשת הרב', ואילו בימינו יש 'אישה-רב'. היו שסברו שכפי ש'מלכה' יכולה להיות אישה-מלך וגם אשת המלך, כך המילה 'רבנית' יכולה לשמש גם ל'אישה-רב'. אבל הפונים לא נחה דעתם מ'רבנית', ולכן הוצעה להם הצורה רַבָּה, שהיא גם צורת הנקבה של שם התואר רַב (כמו בצירוף 'תודה רבה'). נראה שהמילה רַבָּה כבר מצאה לה מהלכים בציבור כצורת הנקבה של רַב.

In other words, despite the fact that Rabbanit is the traditional term, people didn't like Rabbanit because its common usage is to identify the wife of a rabbi, rather then a female rabbi.  On the Hebrew wikipedia page for female rabbi, (based on this Haaretz article) there is a claim that the Academy had changed its mind due to pressure, and in the past insisted that Rabbanit is the correct word. The feminist religious movement Kolech קולך voted two years ago to adopt the term Rabba, over other suggestions. 

Regardless of the academic discussion a search on Google in Hebrew, was only able to find a single reference to a female rabbi as "rabba" - outside of articles discussing what a female rabbi should be called. This might be because the hebrew רבה is such a common word (Meaning "alot") and in its most common usage "תודה רבה".  However my own guess is that outside of trendy feminists  the term has yet to make much headway.