SAR is an abbreviation for either Storage Aspect Ratio or Signal Aspect Ratio. Since one refers to the ratio of vertical to horizontal Pixels, while the other is based on the shape of a Frame when displayed it's best not to use this abbreviation except under a few particular circumstances, such as when referring its use in a particular piece of software or hardware. For clarity you can substitute Display Aspect Ratio (DAR) for Signal Aspect ratio or just Aspect Ratio (AR) for Storage Aspect Ratio.
Resizing DVD-Video To Square Pixels
Digital Video Fundamentals - Resolution and Aspect Ratio