My personal definition is the ability to bring a sense of authentic existence to a fictional character on the printed page. For me good writing is mainly about the people involved -- yes even erotica. No matter how hot the sex you describe is, no matter how beautifully crafted the storyline is, if these things are not happening to someone i can relate to/care about, it will still fall flat.

Just my tuppence, your mileage may vary.
muse