What is meant by electrostatic energy density ?

Explanation

The energy stored per unit voluem of space is denfined as energy density `u_(E)=(U)/("Volume")`
From equation `u_(E)=(1)/(2)((epsilon_(0)A))/(d) (ED)^(2) =(1)/(2)epsilon_(0)(Ad)E^(2)` or `u_(E)=(1)/(2)epsilon_(0)E^(2)`