1) Elements 90 to 103 (after actinium)
2) The fourteen elements in the bottom row of the inner-transition elements of the periodic table that follow the element actinium (Ac #89). Some reference sources include actinium in this series others do not. For these elements the 5f orbital is the filling orbital. This series is a sub-series of the transition metals.
3) The row of elements below the periodic table, from thorium to lawrencium.