What term refers to any pressure less than atmospheric pressure?

The term that refers to any pressure less than atmospheric pressure is "vacuum." In a vacuum, the pressure is lower than the ambient atmospheric pressure surrounding an object or a system. Vacuum can be defined quantitatively as the difference between atmospheric pressure and the existing pressure within a confined space.

When a space is partially evacuated, it creates conditions where fewer air molecules exist, resulting in a lower pressure relative to atmospheric levels. This concept is critical in various fields, including physics, engineering, and even applications in the fire service, where understanding pressure differences can affect firefighting strategies and equipment performance.

The other terms can describe instances or conditions related to low pressure but do not always convey the precise definition as effectively as "vacuum." For example, while "negative pressure" is commonly used in certain contexts, it might relate more to specific applications like ventilation systems or biological safety. Moreover, "low pressure" is a broad term that does not specify the comparison with atmospheric pressure. "Sub-atmospheric pressure" is another way to describe vacuum but is less common in everyday discourse compared to simply stating "vacuum." Hence, "vacuum" is the most straightforward and widely recognized term that accurately describes any pressure state below atmospheric levels.