Well, I've spoke about this many times and discussed this with many people. To put it simply a case is a service request made by an employee, citizen, or customer which much be resolved by a knowledge worker. Now that sounds simple enough. Let's look at the three things I just mentioned and you decide what it means to you. I'm open for discussion here. I'm just one guy with an opinion who struggles with this from time to time.
This simply put is a request for some type of decision. I request to work on an international assignment in Australia. That is an employee service request. You request for a home mortgage from your bank. That is a customer service request. My wife requests for benefits from the state. That is a citizen service request.
This is the person or persons responsible for determining the outcome of the service request. This have many forms and many levels of experience.
This is not mentioned above but is anything that is needed to support determining the outcome of the service request. This may be legacy information, verbal information, unstructured content, pictures, notes, etc.
Now, that is a quick down and dirty definition. Let's see if you think this makes sense and how you can better refine this course thought.