IT is likely to be the militant group with the most number of names and acronyms.
ISIS, ISIL and IS are used to refer to the group that has made significant territorial gains in Iraq and beheaded several Western hostages on camera. But there are also variants such as Daiish and Daesh.
So what does each of these mean? Or are they all the same?
Islamic State in Iraq and Syria (ISIS) versus Islamic State in Iraq and Levant (ISIL)
The full Arabic name of the militant group is al-Dawlah al-Islamiyah fi al-Iraq wa al-Sham, or Daiish in short. It's the last word in the name - al-Sham - that has created the English debate between ISIS and ISIL.
Al-Sham is an old Arabic word that refers essentially to a chunk of the western Middle East near the Mediterranean. This includes not just Syria but also Israel, Jordan, Lebanon and the Palestinian territories, and even a part of south-eastern Turkey. That territory roughly overlaps with the territory that Europeans labelled the Levant - which is why the Arabic name is sometimes translated as the Islamic State in Iraq and the Levant (ISIL). Al-Sham can also be translated as Syria or Damascus. That's why the group is also called the Islamic State in Iraq and Syria (ISIS).
The British and the US governments are among the countries that use ISIL, but ISIS is said to be more commonly used by countries and news organisations.
But ISIS poses an issue for the many companies and brands that are named after the ancient Egyptian goddess of the same name. In the United States, an American woman called Isis Martinez unsuccessfully petitioned the local media asking them to stop using her first name.
Islamic State (IS)
In June this year, the militant group rebranded and declared itself an Islamic caliphate, changing its name to simply, Islamic State (IS). Its leader Abu Bakr al-Baghdadi was proclaimed as Caliph or the head of the state.
But opponents say a militant group shouldn't be called "Islamic" or a "state".
"The word 'state' implies a system of administration and governance," said David L. Phillips, director of the Peace Building and Rights Program at Columbia University. "It's not a term that would be used to characterise a terrorist group or militia that is merely rolling up territory."
Egypt's top Islamic authority, Grand Mufti Ibrahim Negm, had called on the international community to refer to the group as "Al-Qaeda separatists" and not the Islamic State.
The Islamic Society of Britain and the Association of Muslim Lawyers said in a letter to British Prime Minister David Cameron: "It is neither Islamic, nor is it a State. The group has no standing with faithful Muslims, nor among the international community of nations."
Daiish, Daash, Daish, Daesh
There are different acronyms of the Arabic name in Latin characters, including Daiish, Daash and Daish.
Israel and Arab states use the acronym Daesh, which is seen as derogatory. France followed suit in September by adopting the name in official communications.
"This is a terrorist group and not a state," French Foreign Minister Laurent Fabius said then. "I do not recommend using the term Islamic State because it blurs the lines between Islam, Muslims and Islamists. The Arabs call it 'Daesh' and I will be calling them the 'Daesh cutthroats.' "
US Secretary of State John Kerry started using Daesh and ISIL interchangeably in October during a trip to Egypt, the media noted. In his extensive remarks on the militant group, Mr Kerry used Daesh five times and ISIL 10 times.
The militant group reportedly hates the acronym and is said to have threatened to cut out the tongues of anyone who uses Daesh publicly. Some analysts say the dislike of the term comes from its similarity to another Arabic word Das which means to trample or crush.
SOURCE: BBC, ASSOCIATED PRESS, WASHINGTON POST, TELEGRAPH