Toni,
Your question actually cover more than one back muscle: Romboid, trapezius, lats and dealtoid. Therefore there is more than one exercise to get overall definition on upper back. We have named few of them and I would like to add Rear dealt too.
We work rear dealt working shoulder but as it's the back part of the muscle, working it to the point of gaining visible definition, would give an overall definition you are looking for too.
To sum up
there is more than one exercise to get what you want:
*) one arm row
*) pull up
*) push up, we love and hate but they work back, shoulder,pecs etc it's a compound
*) pull over
* ) Barbell row with a wide overgrip does hit the upper back as well.
It would have been easier if only one exercise could give a result but IMO we we can not. We have to do them all
Being women we do not need to work trapezius much. we *do not* build traps like man.It's just not nice!
Proper bodybuilder do though!
on top of all these we know nutrition play 80%
Here is a useful link:
Ask Nicole: July 12th | FitnessRX for Women
All the best toni,