Proof Of Concept.
This usually is a prototype of some sort, that tests whether the assumptions made during the development stage of the project are valid. It is then used to gather data to further improve the design, so that the design goals of the project can be met.

A POC can be a real-world object, e.g. the first flying scale model of a new aircraft, or something more virtual, like a new design for a computer program.

In the process of going from a good idea to a finished product or technology, the POC is an important step.