The general official term , in USA and Canada is
from the point of view of the person who pays for their service:
People such as doctors, dentists, veterinarians, lawyers, accountants,
contractors, subcontractors, public stenographers, or auctioneers who
are in an independent trade, business, or profession in which they
offer their services to the general public are generally independent
from the point of view of the person paid.
If you are an independent contractor, you are self-employed.
Whether these people are independent
contractors or employees depends on the facts in each case. The
general rule is that an individual is an independent contractor if the
payer has the right to control or direct only the result of the work
and not what will be done and how it will be done.
Then depending of the field, you could use a more precise word, such as sub-contractor (building), consultant (I.T), guest (academy: guest speaker, guest lecturer) etc.