Distance is a numerical description of how far apart objects are. In physics or
everyday usage, distance may refer to a physical length, or estimation based on
other criteria (e.g. "two counties over"). In most cases, "distance from A to B" is
interchangeable with "distance from B to A". In mathematics, a distance function
or metric is a generalization of the concept of physical distance. A metric is a
function that behaves according to a specific set of rules, and is a way of
describing what it means for elements of some space to be "close to" or "far away
from" each other.