What is defined as the speed of the air as it moves through the filter media?

The correct choice for defining the speed of air as it moves through the filter media is "Air velocity." This term specifically refers to the rate at which air travels in a given direction through the filter fabric. Understanding air velocity is crucial in the context of air filtration, as it influences the efficiency of the filter and its ability to capture particles. Higher velocities can lead to reduced filtration efficiency if the media cannot adequately capture contaminants due to insufficient contact time.

Media flow, while related, refers more to the volumetric flow through the filter media rather than its speed. Media velocity would typically pertain to the movement of the filter media itself, which is not the focus here. Airflow rate describes the volume of air moving through a space or filter per unit of time but does not directly describe the speed of that air. Thus, "air velocity" is the term that accurately captures the concept of speed through the filter media.