A hydraulic accumulator is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure from an external source. That external source can be a spring, a raised weight, or a compressed gas.

Compressed gas accumulators are by far the most common type. These are also called hydropneumatic accumulators.

External links

What's An Accumulator?

Hydraulic Accumulator - Detailed discussion of various types.