The ATO has published a factsheet entitled "About integrated core processing (ICP)". It explains that ICP is a single system for processing all ATO registrations, forms, payments and accounting transactions - across all types of revenue products. It is the biggest part of release 3 of the ATO change program and when fully implemented will be a significant change in the way the ATO and agents work together.
For more information and a copy of the factsheet, go here
For a copy of Glossary and definitions - Change program, go here