In geography, 'distance' primarily refers to:

'Distance' in geography specifically pertains to how far a place is from another in terms of location. This concept allows for understanding spatial relationships between different areas and the arrangement of various geographic features. It is fundamental in geography as it helps in various analyses, including the measurement of direct paths (like driving distances), proximity, and accessibility between places.

By focusing on the concept of distance, it supports various fields such as urban planning, transportation, and environmental studies, where measuring the exact separation between two points is crucial. Understanding distance can also influence issues like the distribution of resources or the efficiency of travel and communication.

Other options, while related to geographic concepts, do not accurately capture the essence of what 'distance' refers to in this context. Direction, size, and population density pertain to other aspects of geography, such as orientation on a map, the area of land, and the number of people inhabiting a given space, respectively, but do not define the physical distance between locations.