Deallocation is the process of releasing memory that was previously allocated for use in a program. When a program no longer needs certain data or objects, deallocation helps free up that memory, making it available for other processes. This is important for efficient memory management, as it prevents memory leaks, which can slow down or crash a system. In programming languages like C or C++, deallocation is often done using specific commands, such as free() or delete. In contrast, languages like Python or Java use automatic garbage collection to handle deallocation, which simplifies memory management for developers.