Being pedantic, the authorized admission into a country is a visa.

TN status is a special thing. A TN visa is just a non-immigrant visa given to a TN-eligible Canadian or Mexican citizen.

Canadian citizens don't need to receive a visa prior to their departure for the U.S.; they can get a form of "visa on arrival" through their TN classification. Mexican citizens need to get a TN visa in order to be eligible to get a visa on arrival when they arrive.