"Operating systems" in a much narrow sense were invented in the 1950s. I cannot tell you what people meant by the term back then. Then it seems that they accreted features. And then they accreted more features, like a gigantic snowball or avalanche. Ultimately, this all happened because people were lazy and didn't want to repeat themselves -- and there were lots of things to be lazy about. Also, branding and marketing became important. And now an operating system is defined by its user interface guidelines, the artwork in its GUI, what web browser it comes with, and what other software it does/does not ship with. Some people gerrymander the meaning of the word to justify the architectural choices of their favoured operating system.
By the way, being able to parrot the dictionary's definition of a term doesn't mean you know what it means. And knowing what something "means" (that is, knowing how to use it) does not mean you know its dictionary definition. The education system often forces people to remember these things and regurgitate them, which serves only to help you sound convincing in debates.