What defines a vacuum in terms of atmospheric pressure?

A vacuum is defined as a condition in which there is a reduction of atmospheric pressure within a given space compared to the surrounding environment. Specifically, this often implies an absence of some of the atmospheric pressure, meaning that the pressure in that space is lower than the pressure at sea level, which is approximately 101.3 kPa (14.7 psi).

While a vacuum can lead to a significant reduction in air molecules, it does not necessarily mean there is a complete absence of air, as even a small number of air molecules can exist in a vacuum. Thus, it's important to distinguish between the general concept of a vacuum and absolute vacuum, which refers to a state devoid of matter altogether. The idea of negative temperature is not relevant in this context, and high pressure would be the opposite of what characterizes a vacuum. Therefore, the correct definition captures the essence of a vacuum by focusing on the absence of atmospheric pressure rather than the complete removal of air or other unrelated factors.