One way to define philosophy is simply as what philosophers do. And “philosophers,” in a modern context, generally means philosophy professors. This is in some ways a nice definition, because it rules out the possibility of making the absurd-sounding discovery that much of what philosophers do is not philosophy.
Another definition of philosophy is everything that falls outside the boundaries of other disciplines. There are historical reasons why this overlaps pretty well with the definition of “what philosophers (or philosophy professors) do.” Once upon a time, everything was philosophy. Newton’s greatest work, for example, was called “Mathematical Principles of Natural Philosophy.”
Then it gradually became clear that the accomplishments of people like Newton and Lavoisier were very different than the ideas people Aristotle and Descartes and been playing around with, and the term “natural philosopher” got replaced by “scientist.”
The number of disciplines that had moved from philosophy to science gradually grew. Psychology, for example, took a little longer to split off than chemistry or physics (William James is remembered as both a philosopher and a psychologist). Eventually it became clear that “philosophy” had in effect become everything that had yet to split off into a science. I don’t know when this was first noticed, but Bertrand Russell noticed it some time in the first half of the 20th century.
Now as I said, these two definitions of philosophy have a big area of overlap, but it’s important to keep them separate. Here’s a fallacy that can arise from confusing them: “Some things that are not within the boundaries of any other discipline are worth doing. So philosophy is worth doing. So what philosophy professors do is worth doing.”
Now I certainly think there are philosophy professors who do worthwhile stuff. Many of my favorite people are philosophers! But that doesn’t change the fact that much of what philosophy professors do is not worthwhile. Too often, the fact that there is a voluminous literature on a subject shows only that a lot of professors thought an article on the subject would be a good addition to their CV.