"Next day air" typically refers to a shipping service that guarantees delivery of a package to its destination the following day. There are several synonyms that can be used to describe this service and similar ones, such as "overnight shipping", "express delivery", "priority mail", or simply "next day delivery". These terms convey the same urgency and speed of delivery, and are often used interchangeably by shipping companies and online retailers. Whatever the term used, customers can expect their package to arrive quickly and efficiently, making it a popular option for time-sensitive or urgent shipments.