Why are f-block elements called "Inner-transition elements" - Chemistry - Classification of Elements and Periodicity in Properties

Question

Why are f-block elements called "Inner-transition elements"
?

Priya G N · Accepted Answer

The f- block elements are known as
inner transition elements because they involve the filling of inner
sub-shells (4f or 5f) Their electronic configuration has a
f-orbital (or they have valence electrons f-orbitals) These include
Lanthanide and Actinide series in the periodic table

1 Lanthanides: It consists of
elements that follow lanthanum and involve the filling of 4f
subshell

2 Actinides: It consists of
elements that follow Actinium and involve the filling of 5f
subshell

Why are f-block elements called "Inner-transition elements" ?