In the broadest definition, an overpack is a device that contains one or more packages. It provides protection and is often used for the shipper's convenience or to consolidate two or more packages. Shrink-wrap and banding are, by this definition, overpacking materials.

In the context of shipping and hazardous materials (dangerous goods), an overpack is an enclosure used by a single shipper to contain one or more packages into a single unit for easier handling and stowage during transport. Think of it as a "box for your boxes" or a pallet wrapped in shrink-wrap. The primary purpose of an overpack is convenience and protection; it gathers smaller, legally compliant packages—each already properly marked and labeled—into a larger unit. In 2026, safety regulations require that if the markings and labels on the individual inner packages are not visible through the overpack (for example, if you use an opaque wooden crate), the overpack itself must be clearly marked with the word "OVERPACK" and repeat all the hazard labels and UN numbers found on the items inside. This ensures that emergency responders and transport workers are always aware of the high-fidelity risks associated with the cargo, even when it is consolidated into a single bulk unit for shipping efficiency.