A balanced prime sits exactly at the average of its two prime neighbors: the gap below equals the gap above. The prime 53 is preceded by 47 and followed by 59 — both gaps are 6, so 53 is balanced.
Balanced primes are a window into the fine structure of prime gaps: most primes lean closer to one neighbor than the other, so an exact balance is uncommon. It is conjectured (but open) that infinitely many balanced primes exist. Generalizations look at primes balanced over longer windows of neighbors.