Why is an IAS officer called collector in India?

Why is an IAS grade officer called a collector in India? How is the job and 'collector' connected?

Not all IAS officers are collectors. Administrative head of a district is a District Collector, who happens to be an IAS officer. During British Raj, the District Collector was in charge of collecting revenue from a district. That is how the name came to be and it was never modified though the role changed substantially.
                           Where there is commissionarate system in India head of district is known as district collector and where there is no such system enforced then head of a district is known as district magistrate and collector, DM are executive magistrate and have been given jurisdiction of certain matters upon which they act as judges.