Proxemics is the study of measurable distances between people as they interact.
In a social situation, this distance often discloses information about the type of relationship between the people involved.
The first zone, called the intimate zone, is reserved for lovers, children and close family members or friends. This extends from the point of physical contact with the person to about a forearm’s length, or two feet, away.
The second zone, which extends from two feet away to about four feet away, is for friends or associates that are close, but not necessarily on a regular touching basis.
Between four and 12 feet is the zone reserved for acquaintances, people who are familiar but not close. Those closer to the outer edge of this zone are usually the individuals who are least familiar.
From 12 feet to 25 feet is the zone known to be public distance. This is usually the area of space between an audience and a speaker on stage. It is reserved for larger audiences or strangers.
Beyond 25 feet, one is no longer in proximity.
Source: Mike Sheppard, University of New Mexico