Complex sentence: The term'polycyclic' is used only in the study of infinite groups, because for finite groups the above property is just one of a number of equivalent ways to define the class of solvable groups (also called'soluble groups').
Split into simpler sentences: The term'polycyclic' is used chiefly in the study of infinite groups. For finite groups, the above property coincides with, and can be used to define, the class of solvable groups (also called'soluble groups').