after which a perishable
item is no longer fit for consumption or use, at least according to its manufacturer.
Expiration dates are commonly found on food items like dairy products (including milk, cheese and yogurt), eggs, meats and fruit juices. Drugs and medical products usually have expiration dates.
The choice of an expiry date is somewhat arbitrary. For example, it's not as if milk is perfect up until its expiry date and then becomes completely undrinkable. The process of milk turning bad is a gradual one. From personal experience, however, I can say that milk is definately undrinkable four months after its expiry date.