What is Lanthanide contraction? what are the consequences of lanthanide contraction?

The steady decrease in atomic or ionic radii in lanthanides is called lanthanide contraction.
Reason: Poor shielding effect of 4f-subshell
Consequences of lanthanide contraction:
1) Due to lanthanide contraction the covalent nature of La-OH bond increases from La to Lu
2) Due to lanthanide contraction, the basic character of oxides and hydroxides decreases from La(OH)₃ to Lu(OH)₃
3) Due to lanthanide contraction, second and third transition series elements are almost similar size so it is very difficult to separate pair of elements