CSI defined:

Continual Service Improvement (CSI), defined in the ITIL continual service improvement volume, aims to align and realign IT services to changing business needs by identifying and implementing improvements to the IT services that support the business processes. It incorporates many of the same concepts articulated in the Deming Cycle of Plan-Do-Check-Act. The perspective of CSI on improvement is the business perspective of service quality, even though CSI aims to improve process effectiveness, efficiency and cost effectiveness of the IT processes through the whole lifecycle. To manage improvement, CSI should clearly define what should be controlled and measured.