|See also:||warning, squick|
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Triggers are things that cause a strong, heavy emotional response in a person. These usually occur after something traumatic has happened to them. The term originated with doctors treating war veterans for post-traumatic stress disorder, and is often used by fandom in the labeling or discussion of fanworks.
Triggering material has the potential not only to remind a person of a traumatic event, but to make them feel they are experiencing it again, in detail.  Some common experiences that can be later subject to triggers include sexual assault, physical or emotional abuse, phobias, self-harm, and addiction. Visual media such as vids and animated gifs may also require warning for physical triggers. Quick cuts and flashing lights, for instance, may trigger seizures or migraine headaches.
Trigger has also come to have a colloquial meaning; anything that one dislikes or is squicked by.
Triggers and trigger warnings can be a controversial topic. (People who don't warn, people who demand warnings the author doesn't agree with, people who don't believe in triggers, people who don't want to be responsible for another person's mental well-being, people who want to be responsible for their own mental well-being, "how do you survive in the real world if you're such a delicate snowflake," the misuse of the term, etc.)
In response, some fans include a warning policy on their journals or archives stating what, if anything, they warn for. Some authors who don't include warnings invite potential readers to contact them directly for more detailed information about a story if they are concerned about potentially triggering content.
Content warnings have been common in media for decades: public television's "viewer discretion is advised," radio and television journalism's "some of what you're about to hear/see is graphic," and various music, TV and film labeling.
A trigger, by that name, is a psychiatric term for sensory input of any kind that sets off disturbing memories or emotions. It was originally applied to war veterans, later to other clients who were believed to have repressed memories of severe early childhood abuse. The purpose of therapy was to recover these memories and process them in an atmosphere of trust. This "recovery movement" in psychiatry began in the mid-1980s; it is controversial, with many adherents and detractors.
People undergoing this therapy congregated on bulletin board systems pre-Internet to share experiences. It was probably on one of these forums that trigger warnings first appeared. Usenet discussion groups, notably alt.sexual.abuse.recovery and alt.support.dissociation and the IRC channels associated with them, employed and enforced trigger warnings along with "spoiler space". Certain words were forbidden unless a star or "splat" blotted out certain vowels so that the actual words were not seen. There were constant lengthy discussions about who had triggered whom and which members were "unsafe". Some members became impatient with this system and formed other groups with fewer or no such restrictions. However, this also could create problems in terms of a few members using the less defined protocols as an excuse to bully and dogpile onto others.
Use of the term "triggered" to mean "any negative thought or reaction" also dates back to the 1980s and may have originated with doctors in the recovery movement as they sought to identify clients' painful memories.
History in Fandom
When the concept of triggers entered media fandom is not clear. Some fanzines would often provide brief summaries of stories. Death stories were particularly identified in ad listings or flyers. Adult material usually required an age statement. Go to the warnings section for an in-depth discussion the history of warnings in fandom.
One early use of the term was on alt.tv.x-files.creative: "I do not like rape stories. I do not read them. Luckily, they do not trigger me (though I'm sure they do others), but I don't like them. I would like to have been warned about this before I started." 
In 2000, fans on mailing lists were discussing triggers and how to alert readers to possible triggering content.
"I tend to argue passionately in favor of no warnings, or generalized warnings...and warn more specifically anyway. But I'm never sure where the line is. If I have a story that's got no "obvious" triggers (rape, incest, death, etc.) but the lead character is, say, a bigoted asshole, do I warn people that his attitudes are offensive? Or if I'm writing from the perspective of a mentally ill character, do I warn people that *that* might upset them (and yes, both of these examples are taken from stuff I've actually written in other fandoms). I mean, I know for a fact that that *would* wreck the story for many folks out there. Is it common courtesy, or is it political correctness? Where's the line?"
In 2014, a writer for the Buzzfeed online journal wrote an article about the history of trigger warnings and touched briefly on the use of the phrase in media fandom. She traces an early use to 2002 on Livejournal. She distinguishes the phrase "trigger" from the broader and more general usage of "warning" although acknowledges the two often are used together interchangeably.
Awareness of the term and its meaning grew in livejournal fanworks fandom. In the late 2000s, there were several rounds of trigger warning debates; see Trigger Warning Debate (2009) and Vividcon/Vividcon_2010#The_Debate. As of 2011, much attention was devoted to triggers and trigger warnings (see, for example, the numerous rants at FFR about the lack of trigger warnings and/or the wrong kind of warnings).
The Differences Between Triggers and Squicks
Squick is a fun term that was often used as both a noun and a verb. Either X was one of your squicks, or X squicked you, or squicked you out, or squicked you hard. It was often used in fic exchanges. They would ask for a list of your squicks so that the gifting author would know not to include any hint of them. It was also used in casual conversation with fandom friends, authors, artists, etc. It could be left in comments, or as a reason you just didn’t read your best fandom friend’s latest fic. “Sorry, bff, you know I love your writing, but you have X tagged at the top, and that just squicks me out.” “Hey, no worries, best reader friend! I totally get it. Give this one a pass, but I’ll send you a note when I post my next one! I promise it will be totally X-free! ”Here’s the thing though. In your example, you explain why X is your squick with Y. But the beauty of squick was that (at least in my experience) no explanation was necessary. Not only was it not necessary, it was rarely asked for. A squick is a squick, and there doesn’t have to be any rhyme or reason. In fact, why would you have a rational, bullet-pointed, well-thought-out argument as to why something squicked you out? Very often it’s a visceral reaction, and if you don’t like the thing, you’re likely not going to sit and do deep meditation on why not. Squicks were respected by fandom. You don’t like the thing, okay, we will tag the thing appropriately, you do not have to read the thing, no judgments on either side. There was no fandom policing, only respect. And this, I think, is super important, because fandom policing is a problem, especially when it comes to triggers. “Trigger” has become so overused, so all-encompassing, that people feel they have to defend their legitimate triggers. If X triggers you, it triggers you, and you DO NOT need to provide an explanation. But because “trigger” is so often used in place of “squick,” some people feel they have the right to “call out” those who use the word. They want explanations, they want you to tell them what that triggering concept does to you, so they can call bullshit and feel superior. You don’t have to explain either your squicks or your triggers, but using the correct word stops the fandom police from feeling as though they have the right to ask. Bring “squick” back, people. Don’t devalue triggers, which are horrible, nasty, dangerous things. 
- How The "Trigger Warning" Took Over The Internet, a Buzzfeed article detailing trigger warnings and the role that Livejournal played in popularizing the practice, posted May 4, 2014
- Trauma trigger at Wikipedia
- How was squick used?, Archived version (2016) ("Squicks were respected by fandom. You don’t like the thing, okay, we will tag the thing appropriately, you do not have to read the thing, no judgments on either side. There was no fandom policing, only respect......“Trigger” has become so overused, so all-encompassing, that people feel they have to defend their legitimate triggers....Bring “squick” back, people. Don’t devalue triggers, which are horrible, nasty, dangerous things.")
- I would like to talk with ya’ll about trigger warnings. (Accessed June 25, 2011)
- NEW While We Burned by MulderPhile (NC-17, MS) 1/1), October 4, 1998
- post to the Bindlestitch mailing list dated May 14, 2000, quoted anonymously with permission.
- The madness of me (How was squick used? Like would you tag something...), Archived version (2016)