Define lanthenide and actinides

lanthanides

any of the series of fifteen metallic elements from lanthanum to lutetium in the periodic table (atomic numbers 57–71)

actinides

any of the series of fifteen metallic elements from actinium (atomic number 89) to lawrencium (atomic number 103) in the periodic table. They are all radioactive, the heavier members being extremely unstable and not of natural occurrence

it is the most commonly accepted one