Billing Mediation System is a system which converts datatype of one type to others and it's use usually for billing purposes in telecommunication companies. The term: Billing Mediation System comes from it's begavior: billing mediation software mediate between other system.
In telecommunication companies systems that provide data to the mediation system (called upstream systems) are network elements (eg telephone swithces) and the other systems (called downstream systems) that receive data from the mediation system, perform such actions as archiving, auditing, accounting and generation of bills. So billing mediation systems collect and prepare data in special formats for consumption by downstream systems. Billing mediation system tasks are: transformating formats of data, collection and archive, filtering, conversion, validation, correlation of different input CDRs and many others.
Interesting thing is that non all of data that is transferred by billing mediation system is use for billing purposes. For example billing mediation software can generate traffic statistics based on the number and origin of the records that are passing through the system. The the statistics can be used for any business intelligence module or applications.
Complementary to billing mediation software, comprehensive mediation systems (or platforms) also provide functionality dedicated to Service Provisioning (they frequently intermix as services configured and used by the end customer result in usage data records generation in the network).