What is defined as the space between the front of one man to the back of another?

The term that defines the space between the front of one man to the back of another is "Interval." In military context, an interval is specifically used to describe the lateral spacing between individuals within a formation, ensuring that there is enough distance for movement and communication while maintaining unit cohesion.

In contrast, "Distance" generally refers to the measurement of space between two points, which can apply broadly in various contexts, not specifically to the relationship between individuals in a formation. "Proximity" pertains to nearness or closeness but does not specify the forward or backward alignments needed for a proper definition. Lastly, "Spacing" usually refers to the overall arrangement or distance between units or formations, but may not specifically capture the nuanced relationship described by interval. Thus, interval is the precise term when specifically referencing the distance from one person to the next in a line.