Here's a breakdown:

* Aquifer: A body of rock or sediment that can hold and transmit groundwater. It's like a giant sponge that stores and allows water to flow through it.

* Underground reservoir: While technically correct, this term is more general and doesn't necessarily imply the ability to transmit water like an aquifer does.

So, while "underground reservoir" describes the general concept, "aquifer" is the more specific and commonly used term for a body of rock or sediment that holds and transmits groundwater.