The actinide series is the family of elements with atomic numbers 89 through 102. Actinium is the first in the series of actinides, and the family is named after it.
The asteroids of the Ikalian asteroid belt contained actinides, which provided protection from sensors. Ambassador Kell and Worf therefore deduced in 2367 that Kriosian rebels had to hide in that asteroid belt. (TNG: "The Mind's Eye")