What primary factor distinguishes a unibody frame from traditional body-on-frame structures?

The primary factor that distinguishes a unibody frame from traditional body-on-frame structures is the construction as a single molded unit. In a unibody design, the chassis and the body are integrated into a single piece, which enhances structural integrity and reduces weight. This design helps in distributing crash forces more effectively throughout the vehicle, providing better safety and performance compared to traditional body-on-frame structures, where the body and frame are separate entities that are combined during assembly.

Unibody construction also allows for more efficient manufacturing processes and can lead to a decrease in material usage, although these factors are secondary to the core definition of what constitutes a unibody frame.