What is meant by ‘lanthanoid contraction’?

The lanthanoid contraction is the ‘greater than expected decrease’ in ionic radii of the elements in the lanthanide series from atomic number 57 - lanthanum, to 71 - lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements. This is because atomic radii of 4d and 5d transition elements are nearly same and this similarity in size is due to weak shielding of d-electrons.