Final answer:
The textbook refers to 'Customer decides to buy a shirt' as an external event in business, which occurs when a business interacts with someone outside of the company, affecting supply or demand.
Step-by-step explanation:
According to the textbook section on Identifying Events, the instance where a customer decides to buy a shirt is an example of an external event. An external event is an occurrence that originates outside the business and often represents a transaction where the business interacts with an external party, such as a customer or a supplier. This is in contrast to internal events, which do not involve an external transaction and happen entirely within the business itself.
Economists consider instances like investment or saving decisions as part of inter-temporal decision making because they entail making choices over time concerning when to consume goods. Furthermore, events in economics can affect either the supply or demand within a market. For example, good weather can have an impact on the supply side by possibly increasing agricultural output.