Exactly what is meant by the phrase perishable food?

The word perishable is used to place food into three different categories.  Here’s how Susan Brewer, Ph.D., describes them:
 
1) perishable foods: These are characterized by rapid microbial or  enzymatic deterioration, so  refrigeration is required. 
 
2) semi-perishable foods:  This group includes a) those that contain natural inhibitors, for example, some cheeses  and eggs; b) those that have received some minimal preservation treatment (such as pasteurization) that gives them greater tolerance to environmental conditions.
 
3) nonperishable foods (which remain stable at different temperatures). This category includes cereal, sugar, nuts, soft drinks, dry mixtures, crackers, and so on. 
Foods that we commonly call perishable are those that we generally keep in the refrigerator or freezer. 
 
Source:
Susan Brewer, Ph.D., University of Illinois, Department of Food Science and Human Nutriton