Whether the visitor is a professor or researcher coming to the College or Institute to give a talk/lecture/presentation or whether the visitor is an artist performing on campus or whether the visitor is a consultant working on a project, anybody providing a service such as above is considered an independent contractor.
Payments made to an independent contractor are generally taxable. Below is detailed information how different types of payments are treated tax-wise for U.S. citizens, Legal Permanent Residents, Resident Aliens, and Non-Resident Aliens.
For information on what forms are needed in order to hire an independent contractor see the independent contractor website. For information on how to pay the independent contractor check this voucher checklist.
For international visitors please keep in mind that the visitor is coming on an appropriate visa status. Here is more information on types of visa.