20 Killer Words Every True Crime Buff Should Know

Whether we're preparing ourselves for the worst or we just love a good mystery, many of us can't get enough true crime content. True crime intersects with different fields like law, medicine, and forensics—all of which have their own vocabularies that can be hard for people on the outside to decode. Before binging The Staircase or Making a Murderer, brush up on these important terms every true crime fan should be familiar with.

1. COLD CASE

In legalese, a cold case describes a crime that remains unsolved, but isn't being actively investigated due to lack of evidence. The murder of JonBenét Ramsey, the D.B. Cooper hijacking, and the Jack the Ripper killings are all famous examples of cold cases.

2. LATENT PRINT

A latent print is a fingerprint made of the sweat and oil from one's skin (rather than blood or something more visible). Crime scene investigators usually need powders or chemicals to identify this type of print.

3. BLOOD SPATTER

Blood spatter is the pattern of bloodstains left at a violent crime scene. It's such important evidence in murder cases that there's an entire area of forensics dedicated to studying it. In theory, by analyzing the pattern of blood stains in a crime scene, the investigator can determine important details about the crime committed. But in recent years blood spatter analysis has been criticized. A 2009 report declared that it can give useful information about certain aspects of a crime, but that “the uncertainties associated with bloodstain pattern analysis are enormous” and that “the opinions of bloodstain pattern analysts are more subjective than scientific.”

4. PETECHIAL HEMORRHAGING

Cause of death isn't always obvious in murder cases. When looking at a potential strangulation victim, investigators examine their eyes for petechial hemorrhaging, or the tiny red dots that appear as a result of bleeding beneath the skin. Petechial hemorrhaging isn't a sure sign that someone has been choked to death, but it is likely to appear when the blood vessels in someone's head have been subjected to severe pressure.

5. MASK OF SANITY

Ted Bundy, Charles Manson, John Wayne Gacy—these serial killers were famous not only for their crimes, but their deceptively charming dispositions. This is what crime experts refer to as the Mask of Sanity. Coined by psychiatrist Hervey M. Cleckley in his 1941 book, this describes the phenomena of psychopaths easily blending in with their peers because they don't typically suffer from more noticeable mental symptoms like hallucinations and delusions.

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6. MACDONALD TRIAD

The phrase Macdonald Triad first appeared in a paper by J. M. Macdonald published in a 1963 issue of the American Journal of Psychiatry. It refers to the three behaviors that, if exhibited in childhood, may indicate one's tendencies toward violence later in life. Those behaviors are animal cruelty, fire-starting, and chronic bed-wetting. While many prominent murderers have checked these boxes, experts today are skeptical of using this as a metric to identify future serial killers.

7. RIGOR MORTIS

Rigor mortis occurs when a body stiffens up a few hours after death as a result of calcium build-up in the joints and muscles. This can last a few days, and is one of the clues crime scene investigators use to determine when a murder took place.

8. ANGEL OF DEATH

"Angel of death" is the name given to medical professionals who intentionally kill their patients. In some cases the killer has convinced themselves they're helping the victim by choosing to end their life, which is why they're sometimes called "angels of mercy."

9. BLACK WIDOW

Female murderers are rare—they comprise just 15 percent of serial killers—but not unheard of. Women who commit murder are sometimes dubbed Black Widows after the spiders that devour their own mates after copulating. The moniker is usually reserved for a woman who targets people close to her, kills for personal gain, or uses her femininity to her advantage when committing the crime.

10. LUMINOL

Luminol is a chemical that emits a blue glow when mixed with a certain oxidizing agent. One of the substances that triggers this reaction is hemoglobin, the oxygen-carrying protein found in red blood cells. By spraying a violent crime scene with luminol, investigators can detect traces of blood that aren't visible to the naked eye.

11. GLASGOW SMILE

The Black Dahlia murderer mutilated Elizabeth Short before leaving her remains in a Los Angeles park in 1947. The wounds that would come to symbolize the case were two cuts connecting her ears to the corners of her mouth, giving her the appearance of a perpetual grin. Dubbed a Glasgow Smile because of its prevalent use among Scottish gangs in the 1920s and '30s, this mark has appeared in numerous murder cases since.

12. GUNSHOT RESIDUE

Gunshot residue (GSR) is made up of the propellant particles that are discharged during a gunshot. It often settles on the clothing of anyone who was within a few feet of a fired gun, and it can be an essential piece of evidence when connecting suspects to a crime.

13. BRAIN FINGERPRINTING

Recently featured in season 2 of Making a Murderer, brain fingerprinting is a relatively new practice in crime investigations. After a suspect is hooked up to a helmet that senses brain activity, they're given details about the alleged crime that only the perpetrator would know. If they recognize what's being described, the sensor is supposed to pick up the telltale electrical signals in their brain. While what research there is suggests that the technology may be more reliable than a polygraph test, there still haven't been enough studies to prove its validity.

14. JOHN/JANE DOE

In the world of crime, John or Jane Doe are the names given to a murder victim whose identity is being concealed from the public. These names are often used as placeholders in court cases.

15. MÜNCHAUSEN SYNDROME BY PROXY

Like Münchausen syndrome, people with Münchausen syndrome by proxy manufacture trauma to gain sympathy—but instead of harming themselves they choose people who are close to them as their victims. People with this condition might intentionally make their children sick or disabled, which sometimes leads to their death.

16. COPYCAT CRIME

A copycat crime occurs when the perpetrator is inspired by a different crime, whether it was depicted in a book, movie, or TV show or it happened in real-life. Investigators sometimes have trouble distinguishing between copycat killings and the acts of a single serial murderer.

17. TROPHY

Many serial killers collect "trophies" from their victims after committing a crime. These can be fairly innocuous, like jewelry and footwear, or as gruesome as body parts. Ed Gein—the real-life inspiration for the novel and movie Psycho—used the human souvenirs he kept from his murders to make clothing and furniture.

18. BALLISTICS

Ballistics is the study of the mechanics of firearms. In forensics, this science can help investigators identify gun deaths and determine where and how the weapon was used in a crime—and possibly who pulled the trigger. Though, in the past few years, the ability of ballistics to provide a definite answer has been called into question, with courts preferring "more likely than not" statements.

19. FORENSIC ENTOMOLOGY

One of the more unusual careers for someone who studies bugs is that of a forensic entomologist. These scientists look at how insects interact with crime scenes. Based on what type of bugs are hanging around a corpse and which stage of development they're in, a forensic entomologist can help investigators determine a time of death.

20. LOCARD'S EXCHANGE PRINCIPLE

Put simply, Locard's exchange principle is "with contact between two items, there will be an exchange." Twentieth-century forensic scientist Dr. Edmond Locard came up with this idea after observing that criminals will almost always bring something into the crime scene with them and leave something behind, providing valuable evidence to investigators.