Proxemics is the study of measurable distances between people as they interact. The term was introduced by anthropologist Edward T. Hall
Edward T. Hall
Edward Twitchell Hall, Jr. was an American anthropologist and cross-cultural researcher. He is remembered for developing the concept of Proxemics, a description of how people behave and react in different types of culturally-defined personal space...

 in 1966. The effects of proxemics, according to Hall, can be summarized by the following loose rule:
In animals, Swiss zoologist Heini Hediger
Heini Hediger
Heini Hediger was a Swiss zoologist noted for work in proxemics in animal behavior and is known as the "father of zoo biology". Hediger was formerly the director of Tierpark Dählhölzli , Zoo Basel and Zürich Zoo ....

 had distinguished between flight distance (run boundary), critical distance (attack boundary), personal distance
Personal space
Personal space is the region surrounding a person which they regard as psychologically theirs. Most people value their personal space and feel discomfort, anger, or anxiety when their personal space is encroached. Permitting a person to enter personal space and entering somebody else's personal...

 (distance separating members of non-contact species, as a pair of swans), and social distance
Social distance
Social distance describes the distance between different groups of society and is opposed to locational distance. The notion includes all differences such as social class, race/ethnicity or sexuality, but also the fact that the different groups do not mix...

 (intraspecies communication distance).