Yield is defined as the ratio of the total units delivered defect free to the customer over the total number of units that entered the system.
First Time Yield
First Time Yield is the ratio of the units of a product that pass through a process step the first time without rework over the number of units entered into the process step.
Hidden Factory is a term that is used to define the non-value adding activities in an operation or a process. E.g. reworks, handoffs or any of the 7 wastes. These activities reduce the quality of the product or create inefficiencies in the process that are not known to the process owners. As the name suggests, these activities are hidden from the view. One would need to learn to identify and understand wastes in a process and then be able to remove them. Lean Six Sigma focuses on unearthing these 'Hidden Factories'.
Alternatively, hidden factory can also be defined as the additional good quality products that would theoretically have been possible if the energy and time spent on non-value adding activities was directed towards making good quality products.
An application oriented question on the topic along with responses can be seen below. The best answer was provided by Vastupal Vashisth on 7th September 2018.