What is meant 'lanthanoid contraction'?

Explanation

Lanthanide contraction phenomenon is associated with the intervenstion of the 4f orbitals which must be filled before the 5d series of elements begin. The filling of 4f before 5d orbita l results in a regular decrease in atomic radii called Lanthanoid contraction. This essentially compensates for the expected increase in atomic size with increasing atomic number. the net result of the lanthanoid contraction is that the second and the third d-series exhibit similar radii and have very similar physical and chemical properties.