A "professional camera" means something different to different people.

A "professional" camera to the camera manufacturers is defined as a camera which satisfies users they identify in different "professions", i.e. people that make their livelihood with their cameras and photographic systems.

Many "professional photographers" have more than one camera. Their primary camera generally needs to be versatile enough so that it can be configured for different purposes, but usability and durability are often paramount.

Versatility is generally solved through an ILC, Interchangeable Lens Camera body, which may be either a dSLR or Mirrorless type (now-days, but that also included large-format-view, range-finder and twin-lens-reflex bodies previously). Several lenses (or many lenses) plus suitable accessories complete the versatility qualities.

Durability is generally solved with a camera body that can withstand the rigors of weather and environment, including wet, dusty and extreme temperatures. Body and lens seals and heavier-duty-cycle components tend to be qualities of general purpose systems for general purpose photography.

Usability generally means attention to the user interface along with controls placement, as well as niceties like user-configurable menus and control reassignment capabilities. Usability also implies extra connectivity and even a bright-viewfinder -plus- larger-LCD-display, with more information in each than in lesser tier cameras.

Usability also includes faster, and more accurate, autofocus, faster shooting speeds, ability to work with gloves, etc.; things which a non-professional may not value as greatly.

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In addition to the primary body, professionals often have at least an additional body, but often several bodies, which form part of their "system". Component interchangeability can be vital between the bodies, even if only partially satisfied.

For a second body it may be an identical body to the primary, or it may be a more complimentary body, like a FF plus a Crop body system, which can both backup and enhance capabilities. Professionals like to have options and abilities which differentiate them from competition as well as capabilities which enhance their personal style.

Camera battery standardization may also come in to play more at the pro level.

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Professional cameras often have safety features, like more robust attachments for straps and tripods, etc., and they also often have 2 - card-slots, which add either redundancy or capacity, as the situation requires.

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Finally, professional and semi-professional bodies/accessories/systems allow access to professional services. Canon Professional Services (CPS) and Nikon Professional Services (NPS) are designed to help professional photographers in ways that are worth the price-of-admission, which is over and above the cost of equipment alone. It's a type of elite club, available only to card-holding subscribers, and only available if you own pro or semi-pro equipment.

Whether it's offering expedited repair service or some free services like cleaning, lubricating and adjusting (CLA), or even whole rooms of spare equipment at pro events, CPS/NPS offers it's highest-tier customers tangible benefits that the best professionals either desire or sometimes require.