Too many folks here are equating a 'pro camera' with a 'pro photographer'. A pro photographer can use the tools available to create a great image. A pro camera is a camera designed with durable components and has a longer R&D cycle compared to a consumer camera. A pro camera should work in any condition, take a licking and keep on ticking. A pro photographer can plot their way through a situation and will select gear based on their experience, not based on what is advertised as 'pro'.
IMHO 5D mk3 is a low end pro camera, with a lot of advanced technology, while the 1Dmk4/1Dx are pro cameras.
There are pro photographers who would never choose a 1D-series for their studio work. Some studio photographers believe that anything short of MFD is not a pro camera, yet a Hasselblad probably can't take the same "licking" that a 1D can.
Furthermore, longer R&D cycles are irrelevant: if Sony releases a FF SLR next week with 80MP and 16 stops of DR (with other expected performance) on a $3k body made from plastic, you can bet a bunch of pros are going to grab one and make money from it.
camera is any device a pro
photographer uses to accomplish pro
fessional work Beyond that, it's a question of which professional camera suits a particular area of photography