What is the beginning of that portion of the runway usable for landing called?

The beginning of the portion of the runway that is usable for landing is referred to as the threshold. The threshold marks the point at which the runway begins and where the aircraft should ideally touch down during the landing phase. This area is crucial for pilots as it serves as a reference for the glide path and landing approach. The threshold is specifically designated in runway markings and can affect the landing performance based on factors such as weight and environmental conditions.

In aviation, understanding the precise location of the threshold is vital for ensuring safe landings, as this influences how pilots interpret their approach and touchdown procedures. While terms like touchdown zone, overrun, and runway safety area relate to different aspects of runway operation and safety, the threshold specifically defines the starting point for landing use on the runway itself.