Indicator
A defined, repeatable measure of something, used to assess a situation, compare places, or track progress towards a goal.
An indicator is a defined way of measuring something so that the measurement can be repeated and compared. It usually has a formula, a data source and a set of parameters, and the value of an indicator is that everyone using it measures the same thing the same way.
In planning, indicators do three jobs: describe the current situation, compare places against each other, and track whether a target is being approached. Accessibility indicators are one family, and there are many of them because accessibility can be measured in dozens of ways.
The part worth understanding is that an indicator is a choice, not a fact. It compresses something complicated into one number, and what it leaves out is decided by whoever defined it. A walking accessibility indicator with a 15-minute threshold says nothing about the person for whom fifteen minutes is not walkable. This is not an argument against indicators, which are what make comparison possible at all. It is an argument for stating the definition and the parameters alongside the number.