A classification, the way in which things can be classified.
A pigeonhole (“class”) of a classification (scheme). For each category there is a definition and an explicit list of all the entities to be placed under (within) the category in question. The label of a category is called its rubric (although “rubric” sometimes is used interchangeably with “category”). For example, “pickup trucks” would be the rubric (label) for a category in a classification of motor vehicles. There would be a definition of what distinguishes a pickup truck from other vehicles, and a list of all the vehicles (for a given model year, for example) which were “officially” to be classified as pickup trucks. A category may be designed to contain only a single item (entity) and thus be a “single-entity category,” or it may be designed to contain a variety of related entities.