Concept:
A capacitor stores electrical energy due to separation of charges on its plates.
Step 1: When a voltage is applied across a capacitor, positive and negative charges accumulate on opposite plates.
Step 2: Due to this charge separation, an electric field is produced between the plates.
Step 3: The energy of the capacitor is stored in this electric field.
\[
E=\frac{1}{2}CV^2
\]
Therefore:
\[
\boxed{\text{Electric field}}
\]