Do I need a Kenyan visa to transfer from Beijing to Cameroon in Nairobi?

No, international flights are only transit, so as long as passengers do not leave the transit area (through customs), they do not need to apply for a transit visa.

Visa refers to the procedure of endorsement and seal of passports or other travel documents held by domestic or foreign citizens by the sovereign organs of a country to show that they are allowed to enter, exit or transit. It can also be said that it is an endorsement certificate issued to them. Generally speaking, a visa is a document issued by a country's entry-exit administration (such as immigration bureau or its embassy or consulate abroad) to approve the entry of foreign citizens.