What do you mean by retention, inversion and racemisation with examples?

Retention of configuration: A process in which the relative configuration of an atom is retained. If the atom in question is a stereocenter, retention of configurationusually (but not always) changes R absolute configuration into R, and S into S.

Inversion of configuration: A process in which the relative configuration of an atom is changed. If the atom in question is a stereocenter, inversion of configuration usually (but not always) changes R absolute configurationinto S, and S into R.

 Racemization refers to the converting of an enantiomerically pure mixture (one where only one enantiomer is present) into a mixture where more than one of the enantiomers are present. If the racemization results in a mixture where the enantiomers are present in equal quantities