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.