What is the term for any pressure below atmospheric pressure?

The term for any pressure below atmospheric pressure is "vacuum." A vacuum is created when the pressure in a given space is reduced below the atmospheric pressure, resulting in a condition where there are fewer air molecules present. This concept is crucial in various applications, including refrigeration and air conditioning systems, where creating a vacuum is necessary for processes such as removing moisture from components or ensuring that refrigerants flow properly.

Understanding the concept of vacuum is also vital for troubleshooting and system integrity. For example, the presence of a vacuum can indicate a properly functioning system or help pinpoint leaks if the expected vacuum pressure is not achieved. This concept plays a fundamental role in maintaining the efficiency and reliability of HVAC systems.

The other terms mentioned, while related to pressure dynamics, do not specifically define a condition below atmospheric pressure. "Suction," for instance, often refers to a scenario where fluid is drawn into a system, implying a pressure differential but not necessarily defining a state of being lower than atmospheric. "Negative pressure" can refer to pressures lower than the reference pressure, but it is not as commonly used as "vacuum" in practical applications. "Pressure," on its own, does not convey the necessary context of being below atmospheric levels.