Which term describes an instance of a program that is currently being executed?

A process is defined as an instance of a program that is currently being executed. When a program is loaded into memory and begins execution, it becomes a process. This includes not only the code of the program but also its current activity, which consists of the program counter, the contents of the processor's registers, and the variables that are in use.

Processes are fundamental to the operating system's ability to manage multitasking, as they allow the CPU to handle various tasks simultaneously. Each process operates in its own memory space, thus providing isolation and security from other processes. In contrast, while threads can also be running instances of a program, they are generally considered as smaller units of a process that can run concurrently within the same memory space of a process.

The other terms, such as tasks and sessions, do not specifically refer to an instance of a program in execution as clearly as "process" does. A task can refer to a more abstract concept of work that needs to be done, possibly incorporating multiple processes, and a session is related to a user's interaction period with the computer, which may involve one or many processes, but does not singly represent an executing program instance.