Because of the ongoing COVID-19 pandemic, the phrase social distancing has recently gained currency in the sense of:
The action of practice of maintaining a specified physical distance from other people, or of limiting access to and contact between people, especially family and friends, in order to avoid catching or transmitting an infectious disease, or as part of a community initiative to inhibit its spread. (The phrase is first recorded in this sense in 2004.)

UNITED WE STAND. (But 6ft. apart).—From a billboard near downtown Tampa, Florida, noting Tampa General Hospital, AdventHealth, HCA Healthcare and BayCare Health System—photograph: Tampa Bay Times (St. Petersburg, Florida) of Thursday 23^rd April 2020:

 

However, social distancing originally designated:
The action or practice of maintaining a degree of remoteness or emotional separation from another person or social group. (The phrase is first recorded in this sense in 1957.)

For example, this is how John Keith Avery (1927-2018), then Senior Inspector in the Police Department of New South Wales, Australia, defined his own use of social distancing in Police—Force or Service? (Sydney: Butterworths, 1981):

Another factor in the occupational socialisation process is the developing tendency for the new police officer and his wife to conceal his occupation from people they meet at social functions in off duty hours. This is defensive conduct, part of the carapace which most police almost subconsciously develop to protect them from the psychological abrasions of their career.

One of the earliest uses of social distancing with reference to the transmission of an infectious disease occurs in this enlightening article by Lauran Neergaard, of the Associated Press, published in several U.S. newspapers on Tuesday 11^th October 2005—for example in the Billings Gazette (Billings, Montana):

Washington (AP)—Quarantine—or some version of it—in a 21^st-century flu pandemic would look very different from the medieval stereotype of diseased outcasts locked in a do-not-enter zone.
President Bush ¹’s specter of a military-enforced mass quarantine is prompting debate of the Q-word as health officials update the nation’s plan for battling a pandemic—a plan expected to define who decides when and how to separate the contagious from everyone else.
“All the options need to be on the table,” said Dr. Marty Cetron, head of quarantine at the Centers for Disease Control and Prevention.
Bush’s comments recall how quarantines were enforced in parts of this country in the 1890s, when armed guards patrolled streets to keep victims of smallpox and other dread diseases confined to their homes.
“The image that perhaps was inadvertently conveyed is really a setting in extreme that’s less likely,” Cetron said. “There’s a whole range of options in the public-health toolbox for ways to achieve this goal of social distancing.”
For three years the CDC has been helping states plan how they would enact quarantines in case of a bioterrorism attack. The instructions stress using the least restrictive means necessary to stem an infection’s spread.
And public health officials expect a U.S. quarantine today to almost always be voluntary, with incentives to cooperate. In case of a horrific outbreak, quarantined areas would get first shipments of scarce medicines.
“I don’t think either the Tennessee National Guard or the U.S. Army and Marines will try to establish a cordon sanitaire around Nashville,” said Dr. William Schaffner of Vanderbilt University, an influenza expert who advises the federal government. “That’s not going to happen.”
Actually, “we practice in this country quarantine every day,” said Health and Human Services Secretary Mike Leavitt. “If a child gets the measles, their mothers are expected to keep them at home.”
Vaccination is the cornerstone of fighting a pandemic, and quarantine-like steps are supposed to be brief, “designed to buy time until we have an adequate supply of countermeasures,” CDC’s Cetron said.
The SARS ² epidemic of 2003 illustrated that “the public will voluntarily comply with measures to both protect themselves and their loved ones”—if doctors make the case that the steps are for their own good, he said.
Legally, “isolation” is the term for separating people who already are sick from others. That happens routinely in hospitals, as they limit access to patients being treated for certain infections.
“Quarantine” means restricting the movement of still healthy people who may have been exposed to an infectious disease, in case they’re carrying it. During SARS, for instance, hospital workers exposed to suspect cases were asked to stay home from work during the respiratory disease’s 10-day incubation period.
States have the primary legal authority to enact quarantines during outbreaks within their borders. Federal quarantine authority involves preventing infectious diseases from entering the country and stopping interstate spread. Expanding that authority to encompass a military role might entail legislation, something lawmakers’ staffs have begun mulling as public health experts downplay the need.

¹ George Walker Bush (born 1946) was the 43^rd President of the United States from 2001 to 2009.
² Short for severe acute respiratory syndrome, SARS denotes an infectious disease (caused by a coronavirus) with symptoms including fever and cough and in some cases progressing to pneumonia and respiratory failure.