Under IFRS 15, a customer is defined as:

1. A party that has contracted with an entity to obtain goods or services that are an output of the entity's ordinary activities, in exchange for consideration.
2. A customer could be a single party or a group of related parties.
3. Customers can take many forms, including individuals, companies, non-profit organizations, or governmental bodies.
4. Not every party that an entity has a contract with is necessarily a customer. For instance, in a contract between an entity and its supplier, the supplier is not a customer.
5. The definition of a customer does not include parties who are collaborating with the entity to share in the risks and benefits of a project (they are not considered customers because they are not obtaining goods or services that are an output of the entity's ordinary activities).
6. Similarly, a party that is acting as an agent, intermediary, or in a similar capacity, may not meet the definition of a customer if their role is to arrange for another party to obtain the goods or services.

Understanding who is a customer is essential for the correct application of IFRS 15 because the standard applies to revenue earned from customers.