Outing means the revelation of a fact about a person (usually a fact about their personal life) that that person had not publicly discussed. The canonical example is publicly revealing someone's homosexuality. It is frequently used in geek circles to describe identifying someone by their legal name (or a name recognisable to family and friends) who prefers to be anonymous or pseudonymous online. This usage is also known in internet slang as "dropping doc(uments)s" or "dox".
Reasons for outing
Outing is occasionally defended as necessary to avoid hypocrisy on the part of public figures (for example, a queer politician who denounces rights for queer people).
More controversially, it is also defended as a way to reveal the "true size" of a particular group of people, by revealing how common it is to be secretly a member of the group.
- "I’m on record as saying that the outing of coffeeandink’s real name by Shetterly, Cramer, and Daegmorgan (among others) is appalling. And I’m also the person who outed trolling sockpuppet “Igor Sanchez” as Los Angeles attorney Lukas Jackson. Are these equivalent actions?"
Finally, outing may be used as a threat to prevent people offering unpopular opinions online. The protection of anonymity or pseudonymity allows many people to post about difficult subjects, and the risk of outing might prevent them from posting.
Problems with outing
Outing is however also often extremely damaging, exposing the outed person to career difficulties, personal conflicts, or violence. This damage affects women disproportionately (see list of reasons for anonymity/pseudonymity below).
- Bloggers have the right to stay anonymous. We're continuing our battle to protect and preserve your constitutional right to anonymous speech online, including providing a guide to help you with strategies for keeping your identity private when you blog.
- Because it is a standard identity- and privacy-protection precaution
- Because they have experienced online or offline stalking, harassment, or political or domestic violence
- Because they wish to discuss sexual abuse, sexuality, domestic abuse, assault, politics, health, or mental illness, and do not wish some subset of family, friends, strangers, aquaintances, employers, or potential employers to know about it
- Because they wish to keep their private lives, activities, and tastes separate from their professional lives, employers, or potential employers
- Because they fear threats to their employment or the custody of their children
- Because it's the custom among their Internet cohort
- Because it's no one else's business
Other reasons include:
- Because they have a career where careful and well-researched expression of one's opinions is the norm, and writing outside their field of expertise or without the backing of extensive research would be a negative in their core community (academics, most commonly, but also journalists and some others)
- Because they want to interact while avoiding their comments being read through the lens of the reputation associated with their real name, sometimes even if positive
- Because they wish to hide their gender or other personal characteristics because of hostility or condescension concerns
- Because they are whistleblowers or otherwise criticising powerful individuals or groups
Although pseudonyms used on, eg IRC are used by nearly everyone and are usually although not always fairly transparent (more like nicknames), anecdotally women in particular frequently choose names that are not marked as female, to avoid online harrassment, exceptionalism and a sexualized environment. See Gender neutral names.
Many bloggers have been outed, sometimes by people who disagree with their arguments and sometimes by journalists.
- In March 2009, Mike Doogan -- a Democratic member of the Alaska House of Representatives -- exposed the identity of AKMuckraker, a female blogger critical of Alaska Governor Sarah Palin, after four months of investigation.  
- In 2007, a hacker posted a falsified account of Kathy Sierra's career along with her real address and Social Security number. 
- In April 2009, the legal name of the blogger using the 'soft' pseudonym "hilzoy" (her legal name was well known by readers) at the blog Obsidian Wings was revealed, apparently unintentionally, by journalist Linda Hirshman in Slate magazine. Slate refused to edit the article after publication, and the article has been, for example, used to link hilzoy's legal name to her blog on Wikipedia 
- In July 2009 the real name of the blogger using the stronger pseudonym of "publius" at Obsidian Wings was revealed intentionally by journalist Ed Whelan of the National Review 
- The exposing of police blogger "NightJack" by journalist Patrick Foster for the British newspaper The Times. NightJack attempted to block this by legal action, but the high court ruled against him. In an post for Times Online's Comment Central , Daniel Finkelstein defended the decision to reveal NightJack's identity supposedly because he revealed the confidences of colleagues -- that this would "promote the fearless revelation of truth and expose hypocrisy."
- In September 2010 Australian political blogger Grog's Gamut was outed by journalist James Massola writing in The Australian newspaper. Massola and his editors justified this by arguing that a government employee should not influence public policy under a psuedonym. 
In the ongoing discussions called Racefail in Science Fiction Fandom and Media Fandom online, pseudonymity and outing have come up a few times, most notably when Kathryn Cramer and/or Will Shetterly outed Livejournaller coffeeandink.
- "For the past three weeks or so, I've been forced to contemplate deleting or locking down this livejournal because of harassment and privacy threats I've been receiving as a somewhat bizarre result of RaceFail09." [...] "Kathryn Cramer has been linking my LJ to my full name on wikis and in other people's blog comments"
- "You are outing me after having claimed you would stop. You are outing me after people have told you it is wrong and dangerous. You are outing me knowing nothing of my circumstances. You are outing me having been warned it could expose me to physical violence, sexual abuse, personal harassment, and professional and personal hardship."
- "Dear People Who Keep Pointing Out That My LJ Is the First Search Result for My Full Name... Yes, it is NOW."
- "You outed me either for criticizing your behavior in discussions of race, or for criticizing the behavior of your friends in a discussion of race. You have proven quite conclusively that it really is unsafe to discuss race in sf fandom. You have proven quite conclusively that sf fandom is pretty much unsafe for anyone who has trusted you or someone you might talk to with information they do not want to reveal. You have done me profound damage. You have done sf fandom profound damage. You do not appear to recognize any of this."
- Journaller and fan bookshop comments:
- "Fandom practices require safe and empowered spaces where fans can be fans without fear of backlash. You always hear the horror stories, but it is important to reiterate that backlash does actually happen, that it has happened to real fans. It has happened to me - I lost my job in 2003 after someone released my real full name on the internet without my permission."
Will Shetterly and Kathryn Cramer's opinions are not represented above because they both deleted their blogs.