Donald Trump typically gets an enthusiastic response from crowds when he rails against U.S. cities that are perceived as safe havens for undocumented immigrants. "My opponent wants sanctuary cities," the GOP presidential nominee told the Republican National Convention in Cleveland, drawing boos.

Trump's assertion is accurate: Democratic nominee Hillary Clinton has long supported sanctuary city policies. A 2015 statement from her campaign noted that she "believes that sanctuary cities can help further public safety, and she has defended those policies going back years." In her acceptance speech at the Democratic National Convention last night, she pledged to build a path to citizenship for working immigrants and said it would be "self-defeating and inhumane to try to kick them out."

But just what is a sanctuary city? Despite being part of the national immigration debate for decades, it's actually a disputed term with no single official definition.

In recent years, Congress has made several attempts to defund sanctuary cities. Every one of them failed, but Republican-led states have had more success. Georgia and North Carolina have both passed laws banning -- and in the former, defunding -- sanctuary cities. Last year, San Francisco voted to remain a sanctuary city, and New Orleans became one this year. Philadelphia established, rescinded and restored its sanctuary status in less than a two-year period.

In all of these places, the definitions of sanctuary cities are slightly different. At the baseline, sanctuary jurisdictions are any city or county with a policy or practice that limits local involvement in federal immigration enforcement. But how exactly they do that -- and to what extent -- varies to a large degree.

One of the most commonly used definitions involves whether local police cooperate with the U.S. Immigration and Customs Enforcement (ICE). When an undocumented immigrant is arrested anywhere, ICE is automatically notified. Under the now-defunct federal Secure Communities program, which launched in 2008, ICE requested local law enforcement to detain undocumented immigrants in jail for longer, to give ICE officers time to arrive and possibly deport them.

Most jurisdictions willingly complied with this, but upwards of 300 refused and were labeled as sanctuary cities. According to The Texas Tribune, the most resistance to detainment came from places in California, New York, Colorado and Florida.

To critics of the federal program, Secure Communities resulted in the unnecessary deportation of otherwise law-abiding residents, eroded the trust between police officers and immigrants, created legal trouble for local agencies and strained their financial resources. From 2008 to 2014, over 1.6 million detainer requests were issued, according to data obtained by the Transactional Records Access Clearinghouse at Syracuse University. (How many of those requests were granted is unclear.)

But in 2014, a federal court ruled that ICE’s detainer requests don’t provide legal basis for holding people and might violate the Fourth Amendment. Since then, the Obama administration replaced Secure Communities with the Priority Enforcement Program (PEP). Under this new approach, ICE will only ask local police to detain undocumented immigrants under “specific, limited circumstances,” and in most cases, it will merely request to be notified when “an individual who falls within the PEP priorities” is about to be released from jail.

Sound vague? That's because it is. The federal government hasn't given guidelines on what "limited circumstances" might cause it to take action.

Some states also have their own laws and definitions for sanctuary cities.

North Carolina, home to some of the strictest such laws, considers jurisdictions to be sanctuaries if they direct law enforcement officials to refrain from gathering “information regarding the citizenship or immigration status, lawful or unlawful, of any individual.” State lawmakers adopted that language in response to new policies in Chapel Hill, Charlotte and Durham that sought to do just that. None of the cities, however, consider themselves to be sanctuary cities.

In Georgia, any jurisdiction that violates the state’s ban on sanctuary cities risks losing state funding or state-administered federal dollars. Similar to North Carolina, the Peach State defines sanctuary policy as any regulation that restricts local officials’ reporting of immigration statuses to federal law enforcement. So far, though, no Georgia city has been stripped of funding because of this law.

But whether places comply with federal immigration officers isn’t the only measure that politicians and policymakers use to classify a place as a sanctuary city.  There are other controversial policies that local governments have adopted to protect undocumented immigrants.

Take municipal identification cards. Places like Oakland, Calif., and New York City issue city IDs to people who don’t otherwise have legitimate forms of identification. These IDs connect people to public services, programs and benefits regardless of immigration status and are usually recognized by local police officers.

That's problematic for law enforcement, according to Jessica Vaughan, director of policy studies for the Center for Immigration Studies, which supports limiting the number of immigrants allowed into the country.

“It creates opportunities for people to create new identities for themselves, and by definition they are not reliable as a form of ID,” she said.

But Wendy Feliz, a spokesperson for the pro-immigrants’ rights American Immigration Council, said attacks on municipal IDs are overblown.

“[Critics say] municipal IDs are creating sanctuary cities, or municipal IDs are amnesty,” she said. “I would say law enforcement generally likes municipal IDs because they actually want people to have identification and to know who people are.”

Still, the most widespread measure of whether a place is a sanctuary city is their cooperation -- or lack thereof -- with PEP. Had the most recent Congressional bill not died, it would have punished a large number of jurisdictions as well as the entire state of California, according to Melissa Keaney of the pro-immigrants' rights National Immigration Law Center.

It would have blocked federal funding from jurisdictions that prohibit their officials from sharing information regarding an individual’s immigration status with federal agents, as well as those who refuse to comply with PEP detainer or notification requests. The first clause of the bill was ambiguous and, if enacted, could have led to confusion over which places to defund.

One year into PEP’s implementation, its longer term effects are yet to be seen. But ICE reportedly issued more than 7,100 detainer requests in October 2015, which is roughly 4,000 less than they did under Secure Communities the previous October.

“We don't know whether [PEP] is an improvement or not,” said Muzaffar Chishti, a director of the Migration Policy Institute, a pro-immigration think tank. “What it is intended to do is to make it easier for the local governments to take part in it than in the past.”

According to ICE, that’s already happening: 17 of the top 25 jurisdictions that most strongly refused to cooperate with Secure Communities are now PEP participants.

While the switch from Secure Communities to PEP was intended to alleviate critics’ concerns, some are still worried about the negative connotation of local law enforcement even notifying the feds of undocumented immigrants.

“That actually creates a perception in the community that police are involved in enforcing immigration laws,” said Keaney.

Pro-enforcement advocates, however, worry that PEP is overly lax and will result in the release of more criminals back to the streets. ICE, under attacks from both sides, maintains that the new program allows them to focus on the most serious public safety and national security threats.

But some cities remain defiant.

Philadelphia Mayor Jim Kenney issued an executive order in January stating that the city will not comply with PEP until the federal agencies “listen to individuals concerned with this new program, and ensure that community members are both informed and invested in the program’s success.”

San Francisco is also upholding its sanctuary policies, which ban city officials from complying with detainer or notification requests unless required by a court order.

At the center of this debate are police officers. In many cities, they must tread a fine line between helping enforce federal laws and building trusting relationships with the community they serve. The Major Cities Chiefs Police Association, which supports PEP’s notification system but remains wary of detaining immigrants, has concerns about local involvement in federal immigration enforcement.

“To do our job we must have the trust and respect of the communities we serve,” wrote president Tom Manger in a Senate testimony. “When immigrants come to view their local police and sheriffs with distrust because they fear deportation, it creates conditions that encourage criminals to prey upon victims and witnesses alike.”