From IPAPI

Process efficiency is the measure of a process's ability to be performed without activities that are non value-add or unintentional.

IPAPI Definition

Process efficiency - The ratio of value-added work to non value-added work plus unintentional work created by a process.

A process that is completely efficient (100% efficiency) creates no unintentional work, nor does it have any non value-added work within it.

Overview

In an ideal world, every one of our processes would never create any unintentional work nor would they have any non value-add work within them. The only activities within the process would be value-add activities, and these activities would be performed flawlessly.

Much like the general concept of quality, a completely efficient process can not exist. However, some processes can get very close to ideal efficiency and process optimization is an activity that continually seeks to drive a process closer and closer to the ideal state of perfect efficiency.

One of the key measures of process efficiency is its causes of work factor. The causes of work factor tells us how many places exist within the process where work is "caused" and where unintentional work can be caused.