Plutonium is a metallic, chemical, radioactive element. It is the detonation charge used in the five remaining nuclear warheads in battlestar Galactica's munitions following the Cylon attack.
Dr. Gaius Baltar asks for the plutonium from one of the warheads from Commander Adama to build his Cylon detector (Bastille Day).
Note
Canonical information from several episodes (noted in detail in "The Farm" by Simon) indicate that the radiation effects from nuclear fallout on Caprica appear identical with the classic radiation poisoning symptoms from plutonium here on the real-world Earth, strongly suggesting that the element is identical in both "universes." By extension, it is likely that Cylon nuclear weapons also use plutonium.