Antimatter pod

Antimatter pods were self-contained magnetized storage devices which contained antimatter fuel for starships. An antimatter containment field was used to isolate the fuel inside from contact with normal matter, thus preventing a catastrophic reaction. (Star Trek: The Next Generation Technical Manual)

In 2370, Deep Space 9 prepared to receive a shipment of 20 antimatter pods destined for the USS Hannibal.

The Enterprise-D had 30 antimatter pods. (DS9 novel: Antimatter)