Definition: The steady decrease in atomic and ionic ($M^{3+}$) radii across the lanthanoid series from $La^{3+}$ (106 pm) to $Lu^{3+}$ (85 pm). Cause: 4f electrons are poor shielders (diffuse, inner orbital), so each additional 4f electron does not fully shield the increasing nuclear charge → effective nuclear charge increases → contraction. Consequences: (1) Near-identical radii of 4d and 5d elements in same group (Zr/Hf: 160/159 pm, Nb/Ta: 146/146 pm). (2) Difficulty separating lanthanoids. (3) Decreasing basicity La(OH)3 > Lu(OH)3. (4) Complex formation tendency increases $La^{3+}$ < $Lu^{3+}$. (5) Coordination numbers decrease slightly across the series.