A miniaturized fusion reactor similar in appearance to the microfusion cells from previous Fallout games. The fusion cells are used by laser weapons. Fusion cells can be purchased from various merchants within the Commonwealth and looted from enemies using energy weapons.
By doing the Call to Arms quest, it is possible to potentially get an infinite amount of fusion cells. At the end of the quest, you have to press a button to stop the waves of synths attacking Paladin Danse. By simply not pressing the button, you can effectively get unlimited waves, and fusion cells with each one (you don't even have to kill the synths as Paladin Danse will likely do it faster than you and that he cannot die during that quest).