By a two-to-one margin, Americans have consistently expressed concerns about new terrorist attacks at home. But there’s a whole lot less agreement on how to define the word "terrorism." Ask 1,000 people, you’ll probably get 1,000 answers.

"Ordinary citizens, the media, and politicians throw around the term ‘terrorism’ so loosely that in ordinary conversation it has lost all but the most vague meanings," said Lance Janda, a military historian at Cameron University in Lawton, Okla. "Some folks use it as a catch-all term to describe attacks or events that they don't like rather than being more precise."

It's ironic -- the word "terrorism" appears constantly in newscasts, congressional debates and speeches by world leaders, often as a way of securing public support for one security measure or another. But for such a widely used word, there's actually no single definition of what "terrorism" means. There are many, and often, they're incompatible.

Not only is the public confused about what to call "terrorism" -- the U.S. government is as conflicted as anyone. When Bruce Hoffman, a terrorism expert at Georgetown University, published the second edition of his book Inside Terrorism in 2006, he found considerable differences in how federal agencies defined terrorism. For instance:

• Unlike other cabinet departments, which focus on attacks on civilians, the State Department includes attacks on "noncombatant targets," which can include cafes and other facilities frequented by off-duty service personnel, as well as military installations that aren’t in the midst of military hostilities.

• The FBI, unlike other agencies, addresses attacks against abortion clinics, medical research facilities and businesses accused of harming the environment. The FBI also explicitly considers private property damage to be terrorism if it’s motivated by ideology.

• The Department of Homeland Security, echoing its mission, emphasizes "critical infrastructure" and focuses on "mass destruction."

• The Defense Department puts more emphasis on the threat as opposed to the actual act of violence, and it, unlike the others, specifically cites religious aims as a rationale.

Confused yet? Get ready for more head-scratching. These four divergent and sometimes incompatible definitions -- coming from different parts of just one government -- represent merely the start of a wandering quest to nail down a definition of what comprises "terrorism."

Peter Spiro, a Temple University law professor, pointed out that the Immigration and Nationality Act includes an extraordinarily broad spectrum of people who can be considered a terrorist for the purposes of immigration law. Terrorism can range anywhere from the "use of any … nuclear weapon or device" all the way down to the "transfer of funds" to a group the donor "knows, or reasonably should know," is a terrorist organization.

In other words, Spiro said, "someone who gives $5 to Hamas' humanitarian arm is considered to be engaging in terrorist activity" -- the same label afforded to someone who sets off a nuclear weapon in Times Square.

And we haven’t even brought up the differences between countries yet.

Steven R. Ratner, a University of Michigan law professor, notes that the United Nations has been negotiating a comprehensive terrorism convention on and off since the early 1970s. One of the problems, he said, is that some countries want an exception for various liberation movements. For this and other reasons, "the convention is basically stuck and going nowhere," he said.

Then there are myriad definitions by non-governmental organizations and academics. In Hoffman’s book, he relates the story of Dutch-born terrorism scholar Alex P. Schmid, who spent four years researching a definition of terrorism. In the first line of the second edition of his book, Schmid wrote, somewhat exasperatedly, that the "search for an adequate definition is still on."

He’s not alone. Another terrorism scholar, Walter Laqueur, concluded that "no all-embracing definition will ever be found, for the simple reason that there is not one terrorism, but there have been many terrorisms, greatly differing in time and space, in motivation, and in manifestations and aims."

Laqueur wrote that centuries ago, "terrorism" had a "code of honor" that targeted kings, military leaders and other high officials; if such a killing endangered the target’s family, terrorists would call off the attack, even if doing so endangered their own lives.

Today, by contrast, "indiscriminate terrorism has become the rule," he wrote. "Very few leading politicians or generals have been killed, but very many wholly innocent people have."

So while terrorists once weren’t bothered by taking on the label, they now want to be known as "a freedom fighter, a guerrilla, a militant, an insurgent, a rebel, a revolutionary — anything but a terrorist, a killer of random innocents."

The power of the word "terrorism," of course, has affected governments’ behavior, too.

Governments like having loose definitions of terrorism for a couple reasons. Having a broad definition (or multiple definitions) increases a government’s options for responding. And once the government does label something "terrorism," it’s a surefire way to justify hard-edged responses to the public.

Sometimes a government will deny something is terrorism because they agree with the ultimate goal. "What is problematic is that too many places and people are not interested in arriving at a reasonable working definition because they are heavily invested in promoting terrorism," said Theodore R. Bromund, a senior research fellow at the Heritage Foundation.

Governments also benefit from the common belief that terrorism is committed by non-state actors, whether that happens to be an organized group or a "lone wolf."

"Some would argue, for example, that Allied strategic bombing during World War II was a kind of terrorism because we targeted civilians in the hope that it would help induce the governments in Japan and Germany to end the war," Janda said. "The answer often largely depends on who you're asking and what axe they have to grind."

Sometimes it’s easier to define what terrorism is not than what it is. Here are some of the notions that Hoffman considers distinct from terrorism:

• Terrorism isn’t ordinary criminal activity. Yes, terrorism can inspire some criminal activities for the purposes of raising money for the ideological cause, and some criminal gangs can use tactics that are as terror-inspiring as any terrorist group. But when a bank robber waves a gun in a teller’s face, Hoffman writes, he is "conveying no ‘message’ (political or otherwise) through his act of violence beyond facilitating the rapid handing over of his ‘loot.’ … The ordinary robber doesn’t care about changing ‘the system.’"

• Terrorists aren’t "lunatics." Terrorists may attempt to assassinate political leaders, but that doesn’t mean every assassin is a terrorist. "The lunatic assassin’s goal is more often intrinsically idiosyncratic, completely egocentric and deeply personal," Hoffman wrote, citing John Hinckley, who tried to kill President Ronald Reagan in 1981 to impress the actress Jodie Foster.

• Terrorists are more than just extremists. A credible threat of violence can qualify under this category, as can an act that aids a subsequent violent act. But a terrorist can’t simply think about violence; he or she has to take action. "Many persons, of course, harbor all sorts of radical and extreme beliefs and opinions, and many of them belong to radical or even illegal or proscribed political organizations," Hoffman wrote, "However, if they do not use violence in the pursuance of their beliefs, they cannot be considered terrorists."

The trickier lines to draw are between terrorism and two types of armed conflict -- guerrilla war and insurgency.

While there is some overlap in methods and goals between these categories, Hoffman writes, guerillas tend to be larger groups operating as a military unit and seizing territory where they exercise some degree of control over the population. Insurgencies do these things, and also coordinate propaganda to mobilize popular support against an established government, imperialist power or foreign occupier.

While Hoffman finds reasons to distinguish between these categories, he acknowledges that governments don’t necessarily agree. At the time his book was published, "nearly a third of the 37 groups on the U.S. State Department’s ‘Designated Foreign Terrorist Organizations’ could just as easily be categorized as guerrillas."

And if anything, the confusion has gotten worse in recent years.

For one thing, "war" has been defined a lot more fluidly since the 9/11 attacks, writes Mary Ellen O’Connell, a University of Notre Dame law professor. O’Connell says such shades of gray have enabled the U.S.since 9/11 to counter terrorism by relying more and more on the law of armed conflict, rather than police work and criminal law -- a shift she opposes.

Janda added that attacks in Iraq and Afghanistan pose particularly thorny definitional challenges. While attacks on U.S. forces are fairly described as insurgency rather than terrorism, some of the very same attacks are also intended to threaten or kill civilians in proximity to or working with U.S. troops. Depending on the specific example, these attacks on civilians might variously be described as terrorism, genocide (if they were done to advance ethnic cleansing), or criminality (if the goal was to steal property or money).

"The debate can get very messy, and at a certain point it can become pointlessly esoteric," Janda said.

Hoffman, for what it’s worth, concluded in his book that that terrorism is "the deliberate creation and exploitation of fear through violence or the threat of violence in the pursuit of political change."

Hoffman’s logic was that "all terrorist acts involve violence or the threat of violence. Terrorism is specifically designed to have far-reaching psychological effects beyond the immediate victim or objects of the terrorist attack. It is meant to instil fear within, and thereby intimidate, a wider ‘target audience’ that might include a rival ethnic or religious group, an entire country, a national government or political party, or public opinion in general."

At least experts take some solace that the lack of an ironclad definition of terrorism hasn’t stopped the U.S. and it’s allies from formulating a counterterrorism policy.

"If you look at counterterrorism efforts between the U.S. and Europe, it’s not as though countries are confused and unable to cooperate effectively," said Daveed Gartenstein-Ross, senior fellow at the Foundation for Defense of Democracies.