noun Any of the 15 radioactive elements of the periodic table from actinium to lawrencium that are positioned under the lanthanides, to which they have similar chemistry.
noun Any of the 14 elements from actinium to nobelium (lawrencium is excluded by some authors as it is a d-block rather than an f-block element).
noun Any of the 14 elements from thorium to lawrencium (actinium having been mistakenly excluded because it was thought to be a d-block element in the past).