POW or "position of wear" is a term that is used to describe the way that an eyeglass frame positions lenses in front of the eye. When the tilt, faceform and vertex distance of lenses change in front of the eye, the effective power of the lens also changes as well as vision through the lens' periphery. It's no longer the actual prescription.
Since virtually every eyeglass wearer has different facial features, asymmetry and varying ear positions, the same frame fits many faces differently. That means that to deliver the right prescription with more precision, POW should be considered. When POW values are considered during the surfacing process, lens powers can be optimized to deliver the best prescription for the differences created by the frame.
Here is a detailed technical paper about POW by Darryl Meister (Carl Zeiss Vision) written as a memo to the Technical Committee of the Vision Council as a review of the opportunity. To deliver the best optical performance in the eyewear that you sell, this is worth a read. Thanks to Zeiss for allowing us to re-print it here.