On November 14, 2015, a set of co-ordinated attacks against civilians in Paris killed 130 people.
It was labelled a terrorist attack.
Last Tuesday a truck ploughed along a Manhattan bike path into a crowd of people killing eight and wounding dozens more.
The driver was labelled a terrorist.
A month ago, Stephen Paddock opened fire on a crowd of concert-goers in Las Vegas killing 58 people and injuring 546 others.
However, it was not labelled a terrorist attack.
On Sunday a 26-year-old US Air Force veteran and Bible study teacher opened fire inside a Texas church killing at least 26 - including a 5-year-old.
It wasn't labelled a terrorist attack either.
The latest spate of attacks in the US set off a round of fierce debate.
On the surface, this could be considered a straightforward question of motive.
Not every killing or mass shooting is terrorism - but in a new generation of religious attacks and the emergence of Isis the line between what is terrorism and what is a disturbed lone wolf has been blurred.
Recently, mass shooters have shown signs of both mental illness and attachments to ideological causes.
But what is terrorism?
Terrorism is described as the unlawful use of violence and intimidation, especially against civilians, in the pursuit of political purposes.
In other words, without any knowledge of an attacker's motivations, authorities are unable to officially call something an act of terrorism.
"We have the tendency to label anything we abhor as terrorism," Bruce Hoffman, an expert on terrorism who serves as Georgetown University's director of security studies, told Business Insider.
"But the fact is, even if it may cause terror and generate profound fear and anxiety, it's the political motive that is salient in determining whether it's an act of terrorism."
Despite Isis claiming responsibility for Stephen Paddock's Las Vegas attack, there was no direct link to the terrorist organisation, which is why he was not labelled a terrorist.
So why isn't Sunday's mass shooting where 26 people died not being called a terrorist attack?
Shooter Devin Patrick Kelley did not have any apparent political motivation behind his deadly assault.
But what is known is that he was recently married, he had recently posted a photo of an AR-15 style gun on his Facebook page with the caption: "She's a bad b***h".
He joined the US Air Force after graduating New Braunfels High School in 2009. He worked in logistics and supply in the Air Force until 2013, when he left and volunteered as a teacher for Bible studies at Kingsville First Baptist Church.
Investigators said Sunday's church massacre in Texas occurred amid an ongoing "domestic situation" involving the gunman and his relatives, at least one of whom had attended the church.
The Church pastor's 14-year-old daughter was also among the dead.
But there was no apparent political motivation behind the massacre, hence why the attack is not labelled terrorism.
However, the label can easily change as investigators uncover more details about an attacker's history and potential clues into the person's political ideology.
Massey University researcher Dr John Battersby believes we should be careful about who and what we call terrorists.
Battersby said whether or not a group is considered terrorists can depend largely on the views of the state.
The example was given of Nelson Mandela, who consciously carried out attacks against his country's infrastructure, yet would not be considered by many to be a "terrorist".
However, he said that "people who end up in the terrorism camp are deliberately planning their actions to make a political statement".
"Terrorism usually aims to influence the political environment and the freedom of people to make decisions," he said.
"It is horrendously difficult, it's very subjective ... it very much depends how a state defines a group."