Virtual Reality $($VR$)$:

Virtual Reality $($VR$)$ refers to a computer$-$generated simulation of an environment or experience that is designed to feel as realistic and immersive as possible. VR headsets, along with other hardware and software, are used to create a virtual world that users can interact with using controllers or other input devices.

Industry $4\mathrm{.}0$:

Industry $4\mathrm{.}0$ represents the fourth industrial revolution, characterized by the integration of advanced technologies such as artificial intelligence, the Internet of Things $($IoT$),$ big data, and cloud computing in the manufacturing sector. The goal of Industry $4\mathrm{.}0$ is to improve the efficiency, productivity, and flexibility of manufacturing processes.

Maintainability Approach:

The maintainability approach is a software development methodology that focuses on creating software that is easy to maintain over its lifecycle. This approach emphasizes the use of design patterns, modularity, and automated testing to ensure that software is reliable, scalable, and maintainable.

Final answer: In summary, Virtual Reality $($VR$)$ is a technology that creates a simulated environment that users can interact with, Industry $4\mathrm{.}0$ is a term used to describe the integration of advanced technologies in manufacturing, and the maintainability approach is a software development methodology that focuses on creating easy$-$to$-$maintain software.