Nigerian insurgent group Boko Haram was the world's deadliest “terrorist” organization in 2014, according to a report published Tuesday.

Boko Haram members killed 6,644 people last year in its campaign to overthrow the Nigerian state, according to the Global Terrorism Index, published by the Institute for Economics and Peace (IEP). The IEP study did not include state terrorism: either against local populations or foreign ones.

Comparatively, the Islamic State group was responsible for 6,073 deaths in 2014.

Overall, the IEP's study recorded an 80 percent increase in “the number of lives lost to terrorism” between 2013 and 2014.

“The significant increase in terrorist activity has meant that its ramifications are being felt more widely throughout the world,” said IEP Executive Chairman Steve Killelea.

“What is most striking from our analysis is how the drivers of terrorism differ between more and less developed countries. In the West, socio-economic factors such as youth unemployment and drug crime correlate with terrorism. In non-OECD countries, terrorism shows stronger associations with ongoing conflict, corruption and violence,” he explained.

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In Western countries, most “terrorists” are “lone wolf attackers,” according to the IEP study.

“Islamic fundamentalism was not the main driver of terrorism in Western countries: 80 percent of lone wolf deaths were by political extremists, nationalists, racial and religious supremacists,” the report concluded.

There is no universally accepted definition of terrorism, with the IEP itself conceding, “Defining terrorism is not a straightforward matter.”

According to the organization's website, “The Institute for Economics and Peace uses the University of Maryland START Centre definition of terrorism as ‘the threatened or actual use of illegal force and violence by a non-state actor to attain a political, economic, religious, or social goal through fear, coercion, or intimidation.’”