The difference between the earned value and the actual cost is the cost performance index.

Earned Value

A project management technique for measuring project performance and progress in an objective manner.

Actual Cost

The final, real amount spent on completing a specific task, project, or acquisition, as opposed to estimated or budgeted costs.

Cost Performance Index

A measure in project management that calculates the financial efficiency and cost control of a project by dividing the earned value by the actual cost.