The Zone of Proximal Development (ZPD) defines the readability range within which pupils should read to best develop their reading, while avoiding frustration. Zone of Proximal Development or ZPD is used to describe the relationship. What is ZPD? Product: Renaissance Accelerated Reader, Renaissance Star Reading. ZPD = Zone of proximal development (ZPD) is a range of readability levels from which a student should select books to read. Get in the Zone: ZPD marks every student's sweet spot. An NCE gain of zero represents the national average.