Tritonium is an element.
Bajoran Resistance fighters carried subdermal implants of tritonium isotopes during the Cardassian Occupation of Bajor.
According to Kira Nerys, these implants could be activated, if the individual was captured, and leave a trail of isotopes, which would be used to track and rescue the individual.
In 2372, Kira and Dukat were able to trace tritonium isotopes that were left behind by the Breen-captured prisoners taken from the Ravinok. (DS9: "Indiscretion")
The underground chamber, where the last of the Vaadwaur hid in bio-pods, had walls reinforced with tritonium. (VOY: "Dragon's Teeth")