Which term describes software used to facilitate application communication in a system?

Middleware refers to software that acts as an intermediary layer between different applications or services, enabling them to communicate and work together. This term covers various interfaces and services that facilitate communication, data management, and messaging across distributed systems. Middleware supports the creation of complex applications by helping developers connect their software components efficiently, regardless of the programming languages or underlying systems they use.

In the context of systems architecture, middleware plays a critical role by providing essential services like message queuing, application integration, and transaction management. This ability to orchestrate communication makes it invaluable for enterprise applications, microservices architectures, and cloud environments.

The other options represent distinct components of a computing environment. The kernel is the core part of an operating system that manages system resources, while a hypervisor is software that enables virtualization by allowing multiple operating systems to run on a single hardware platform. A database is used for storing and managing data but does not inherently facilitate application communication. Thus, middleware is uniquely suited to fit the definition provided in the question, highlighting its vital role in application communication.