is someone who is most likely an expert in the specific field which they are writing about. They have either spent a lot of time studying on, around, or in this field, as well as studying the works of other professionals.
This person would have left no stone unturned in their search for the most accurate information, and usually a great deal of respect from their peers and superiors. This is all very important, as it helps to provide that there is truth behind the information, and that it is fairly accurate and trustworthy as well. Which leads us to why it is necessary to use credible sources in our work. It will add strength and "weight" to our writing, allowing our readers some degree of comfort that what they the information contained within is correct.