The term 'POPULAR COMPUTER OPERATING SYSTEM' can be summarized by the four-letter word UNIX. UNIX is a multi-user, multitasking operating system developed in the 1970s at Bell Labs. It is widely used in servers, workstations, and personal computers, and is the basis for many other operating systems. UNIX is known for its stability, scalability, and security, making it a popular choice for many businesses and organizations.