the elements in which electrons are progressively filled in 4f-orbital are called

Lanthanoid are known for their 4 f orbitals so the electrons are also filled in these orbitals.

For example, [Xe] 4f⁷ 5d¹ 6s²= Electronic Configuration of Gadolinium

This is an element present iin the lanthanoid series and has electrons present in the 4f orbitals.

(Diagram of Lanthanoids).