From The Daily Mail:
Police fear a gang-rape phenomenon known as ‘taharrush gamea’ in the Arab world and seen in attacks on women across German cities at the New Year has now spread to Europe.
The name of the practice translates to ‘collective harassment’ and is carried out by large groups of men who sexually assault lone women, either by groping, or in some instances, raping them.
And the attack usually goes unpunished because the large number of perpetrators and chaos of the attack means authorities are unable to identify those involved.