Accessibility
How easily people can reach the things they need from where they are. First defined in 1959 as the potential of opportunities for interaction.
Accessibility is how easily people or goods can be reached from a given location, or how easily people can reach what they need. It was first defined in 1959 by Walter Hansen as “the potential of opportunities for interaction”, and in planning and GIS it usually means the ease of reaching essential services: healthcare, schools, shops, work, green space.
It is a property of two systems at once, which is what makes it useful and awkward. Land use decides where the destinations are and how many people live near them. The transport system decides how long it takes to get between them. Change either one and accessibility changes, so a housing decision is an accessibility decision even when no transport is involved.
Accessibility can be expressed through many different indicators, working at different spatial scales and for different modes of transport. That variety is not a weakness, it is what lets the same idea answer questions about a neighbourhood on foot and a region by rail. But it does mean an accessibility figure is meaningless without the indicator, the mode and the threshold that produced it.