Definition
A hypothetical female average or female generic individual; the common woman.
How People Actually Use It
The female version of an "average Joe". A girl that's rather plain and doesn't stand out in a large crowd.
"Girl #1: Omg! That girl in our second period is sooo plain, like geez all she does is listen to the Top 10 hits playlist and watch gossip girl! Girl #2: I know right! She's such an average Jane."
Source: Urban Dictionary