Interpersonal distance describes how far apart individuals are from one another physically. This distance varies with shorter lengths occurring with friends and longer distances with strangers (you will naturally stand closer to your best friend while talking than with a complete stranger who is asking you the time at the bus stop).
Optimal interpersonal distance describes the comfort zone at which individuals stand near each other. This optimal distance differs between cultures and individuals. The Comfortable Interpersonal Distance Scale is used to measure interpersonal distance preferences in individuals.