Explanation:
Multi-threading is a technique that allows multiple threads to execute concurrently within a single process. A thread is a lightweight sub-process that shares resources such as memory, file handles, and code with other threads within the same process but has its own program counter, stack, and registers.

Multi-threading improves the efficiency and responsiveness of applications by enabling parallel execution of tasks. For instance, a multi-threaded web browser can simultaneously download files, render web pages, and handle user input.

Threads can be categorized into user threads (managed by user applications) and kernel threads (managed by the operating system). Modern operating systems provide robust support for multi-threading to achieve better resource utilization and scalability.

Formal Definition:
Multi-threading is the ability of a process to execute multiple threads concurrently, enabling efficient task execution and resource sharing within an application.

Additional Information:
The advantages of multi-threading include faster task execution, better CPU utilization, and efficient resource sharing. However, it introduces challenges like thread synchronization, race conditions, and deadlocks, which require proper thread management techniques.

Multi-threading models vary across operating systems, with common approaches being the many-to-one, one-to-one, and many-to-many models. Programming languages like Java and C++ provide built-in libraries to support thread creation and management.