The phrase "primary display" refers to the main screen or monitor that you use to view and interact with information on a computer or electronic device. It is the main window where you see everything. Full definition
In fact, many of primary display specs are now 5 to 10 percent worse than the first generation2012 Surface RT that we tested, including Brightness, Contrast Ratio, Contrast Rating for High Ambient Light, Specular Reflectance, and White Point. (displaymate.com)
The talking point of the YotaPhone is its dual displays — an LCD primary display and an E-Ink secondary display. (bgr.in)
Blass was also first to reveal detailed information about the V10 a few weeks ago, claiming that it will feature a 5.7 - inch primary display with Quad HD (2560x1440px) resolution, Snapdragon 808 processor, 3 GB of RAM and more. (neowin.net)