What term refers to the difference in pressure that causes air or gas to flow?

The term that refers to the difference in pressure that causes air or gas to flow is known as "draft." Draft is essentially the driving force that moves air or gases through a system, such as in chimneys or ventilation ducts. When there is a pressure difference between two areas—such as higher pressure in one area compared to lower pressure in another—air or gas will flow from the high-pressure area to the low-pressure area to equalize the pressure. This concept is fundamental in various applications, including heating, cooling, and exhausting systems, where proper airflow is crucial for efficiency and safety.

In contrast, ventilation refers to the overall process of supplying or removing air from a space, which involves draft but is a broader term that includes mechanisms and systems for air exchange, rather than just the pressure difference. A pressure drop specifically measures the decrease in pressure within a system, but it does not explicitly indicate flow unless there is a corresponding draft causing that flow. Flow rate measures the volume of air or gas passing through a space over time, but it does not directly describe the underlying reason for that movement. Thus, "draft" is the most precise term related to the pressure difference that initiates the flow of air or gas.