Final answer:
In a relational database, an order_number is referred to as a record, which is a row in a table that uniquely represents an item of data.
Step-by-step explanation:
In a relational database, an order_number is usually referred to as a record. This term is used because a relational database is structured in tables, where each row represents a single, implicitly structured data item known as a record. An order_number would typically be one of the fields within a record, serving as a unique identifier for that record. For example, in a database table that stores orders, the order_number field would uniquely identify each order, and the entire set of information pertaining to that order (such as the date, customer ID, and item) would constitute a record.