Final answer:
In medical coding, a 'Block' is the term used for three characters that represent a single disease or a group of related conditions.
Step-by-step explanation:
The three characters that represent a single disease or a group of related conditions in medical coding (such as the International Classification of Diseases, or ICD codes) is referred to as a Block (D). A Block is part of a coding system that is used by healthcare professionals to categorize and code diagnoses, symptoms, and procedures. In this hierarchy, a Block contains several categories, each of which might include one or more subcategories corresponding to more specific conditions related to the broader classification that the block represents.