Service Improvement Plan - SIP
Service Improvement Plan (SIP) refers to the steps that have to be taken if there is significant gap in the expected delivery quality of a service and the actual delivery.
The SIP is a concept that 'belongs' to the Service Level Management (SLM) process. SLM is primarily defined in the Service Design volume of ITIL v3, but like many processes in v3 SLM is also documented in other volumes.
The SIP is specifically documented in the Continual Service Improvement volume. It is also referred to when capital budgeting is being discussed (in Service Strategy) with reference to preference decisions.