What is a memory leak?

A memory leak occurs when a program allocates memory for use but fails to release that memory back to the system after it is no longer needed. As a result, the memory remains allocated and unavailable for other parts of the program or system, which can lead to a gradual depletion of available memory over time. This can cause the program to consume more and more memory, potentially leading to performance degradation or even crashes if the system runs out of memory resources.

In contrast to the other choices, which describe unrelated concepts, the correct answer focuses directly on the unintended retention of memory. For instance, securing data in memory, crashing a program, or retrieving lost data are not related to the specific issue of memory not being freed after it has been used. Understanding memory leaks is crucial in programming, as they can severely affect the efficiency and reliability of applications.