A plasma infuser was a handheld instrument used to transfer high-energy plasma.
In 2369, Doctor Reyga was found dead with a plasma infuser in his hand. It had been completely discharged, leading Beverly Crusher to believe he had been killed instantly by the blast. She found it odd that he was still holding it rather than having dropped it after dying, which lead her to suspect murder, not suicide, as Worf had suggested.
When Nurse Ogawa questioned her decision to autopsy Reyga as cause of death seemed fairly obvious, Crusher persisted, saying, "As far as I know, he could have been poisoned and someone put the plasma infuser in his hand after he died." (TNG: "Suspicions")