It is the rule by military authorities in an emergency, at times when civil officials are deemed to be unable to function. It often involves the suspension of civil rights and the extension of military law.
President Yoon has not been clear about what restrictions will be in place, but the Yonhap news agency cited the military as saying activities by parliament and political parties would be banned, and that media and publishers would be under the control of the martial law command.
Martial law often also restricts public protests, strikes and potentially other forms of public gathering.
The military also said that the country's striking doctors should return to work within 48 hours, Yonhap said. Thousands of doctors have been striking for months over government plans to expand the number of students at medical schools.
Those who violate martial law can be arrested without a warrant, local news agencies said.
"Tanks, armored personnel carriers, and soldiers with guns and knives will rule the country," Lee Jae-myung, leader of the opposition Democratic Party, which has the majority in parliament, said in a livestream online.
Martial law is in theory temporary, but in nations where it has been imposed, it has often been extended – and may continue indefinitely in some cases.
South Korea’s Constitution states that the president may proclaim martial law when “required to cope with a military necessity or to maintain the public safety and order by mobilization of the military forces in time of war, armed conflict or similar national emergency.”
The martial law commander, appointed by the president on the recommendation of the defence minister among active-duty generals, has the power to take actions related to arrests of persons, search and seizure, speech and the press, and assembly.
The commander has authority over all administrative and judicial matters
Why did the president declare martial law?
Mr Yoon has previously spoken about pro-North Korean, anti-state forces hindering his agenda, without going into detail. It is a simple threat to cite, given the fact the two nations have not officially been at peace since the Korean War ended in an armistice in 1953. Decades of animosity have followed, with repeated military drills, rocket firings and nuclear threats. Pyongyang has recently stepped up its ties with Russia, sending thousands of troops, missiles and ammunition to support Moscow's invasion of Ukraine, a development that has deeply worried Seoul at a point when relations between the two nations are at a low ebb.
The bigger factor may be the domestic political scene for Mr Yoon, who has been a lame duck president since the last general election was held in April and the opposition Democratic Party helmed by Mr Lee won a landslide. When he won the presidency in 2022, Mr Yoon – a member of the People Power Party, beat Mr Lee by just 0.7 percentage points. This was the closest election since South Korea started direct elections in 1987.
Mr Yoon's ratings in the polls have been low, while he has faced a number of controversies, including his wife, first lady Kim Keon Hee, being accused of stock manipulation by the opposition and a scandal over Ms Kim accepting a luxury Dior handbag. Mr Yoon has said his wife has been demonised and that that the allegations are "exaggerated".
In declaring martial law, Mr Yoon cited a motion by the Democratic Party earlier this week to impeach some of the country's top prosecutors over a failure to investigate the first lady as well as its rejection of a government budget proposal.
South Korea's ministers on Monday protested the move by the Democratic Party to slash more than 4 trillion won (£2.3 billion) from the government's budget proposal. Mr Yoon said that action undermines the essential functioning of government.
The declaration of martial law was condemned by opposition politicians and even the leader of Mr Yoon's People Power Party, Han Dong-hoon, called the decision "wrong" and vowed to "stop it with the people".
What happens now?
Under South Korean law, martial law should be able to be lifted with a majority vote in the parliament, which the Democratic Party controls. A majority of MPs – 190 out of 300 – managed to make it to parliament and a motion passed calling for martial law to be lifted backed by all 190 legislators there. It is unclear how quickly this will happen, as the military insisted on the president being the one to lift the order given that political activity is banned under the martial law order.
A few hours later, Mr Yoon backtracked and said he will “accept the National Assembly’s demand and lift the martial law through a cabinet meeting”.
The opposition party has called for Mr Yoon to resign or face impeachment for his destabilising actions. Mr Yoon didn’t offer an immediate public response to the demand. But his office said senior presidential advisers and secretaries for Mr Yoon offered to resign collectively and the president also put off his official Wednesday morning schedule.
Impeaching Mr Yoon would require two-thirds support in the 300-member parliament. The main opposition and smaller parties hold 192 seats, and 18 members of Mr Yoon’s People Power Party opposed him in the 190-0 vote rejecting his martial law declaration. If impeached, Mr Yoon would lose his powers until the Constitutional Court rules on its validity, with prime minister Han Duck-soo stepping in as acting president.
When has martial law been previously imposed?
Before South Korea moved to direct democracy in the late 1990s martial law had been imposed a number of times.
The last time martial law was declared in South Korea was in 1979, after the assassination of the then-South Korean dictator Park Chung-hee, who had seized power in a military coup in 1961.
In 1980, martial law was fully extended after a group of military officers led by Chun Doo-hwan forced then-president Choi Kyu-hah to proclaim martial law to crush calls by the opposition, labour and students for the restoration of democratic government.
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