What is categorized as professional services?

Could you please elaborate on the definition of "professional services"? Are these services typically offered by individuals or companies with specialized knowledge or expertise in a particular field? Are they characterized by the application of technical, analytical, or creative skills to solve complex problems or meet specific needs? Additionally, are there any specific industries or sectors where professional services are particularly prevalent, such as finance, consulting, or legal services? Finally, what factors might influence the demand for professional services in the current economic landscape?