Jargon

1. A theory of probability based on updating subjective estimates in the light of new evidence; contrasted with the more objective frequentist approach.

2. Probablistic reasoning in general.

3. Good, well done reasoning in general.

Beisutsukai

A fictional secret society of Bayesians.

Belief update

What you do to your beliefs, opinions and cognitive structure when new evidence comes along.

Black swan

In the usage of Nassim Nicholas Taleb, a black swan is a rare event whose magnitude is so high as to impact the average of a series, a characteristic of fat-tailed distributions, as opposed to the thin-tailed distributions such as the normal distributionin which rare events are vanishingly unlikely to have such a large impact.

Blues, Greens

Roman Empire chariot-racing teams that became part of politics. Used in place of real party names. See Mind-killer.

CEV

Coherent Extrapolated Volition

"In poetic terms, our coherent extrapolated volition is our wish if we knew more, thought faster, were more the people we wished we were, had grown up farther together; where the extrapolation converges rather than diverges, where our wishes cohere rather than interfere; extrapolated as we wish that extrapolated, interpreted as we wish that interpreted."

Clever arguer

Someone skilled at writing convincing-sounding arguments for an existing belief. Inventing clever arguments for a belief does not change the truth value of the belief.

Discussed in "The Bottom Line"

Consequentialism

A moral theory that places value on the consequences of actions. Covered in more depth here.

Crisis of faith

What to have when you may have been quite wrong for a long time.

Dark arts

Rhetorical techniques crafted to exploit human cognitive biases. Considered bad behaviour even if the belief you want to communicate is good.

Deontology/ deontological ethics

Deontological ethics (from Greek deon, "obligation, duty"; and -logia) is an approach to ethics that judges the morality of an action based on the action's adherence to a rule or rules. See Wikipedia article on deontological ethics for more. Contrast consequentialism.

EEA

Environment of Evolutionary Adaptedness

An Evolutionary psychology term synonymous with the more commonly-used "ancestral environment". For humans, refers to the state of tribal bands of hunter-gatherers.

Egan's law

"It all adds up to normality." Surprising truths do not make the sky orange and grey; it stays blue.

ETA

Edited To Add (though some would rather you say "Edit:" instead)

EY

Eliezer Yudkowsky

FAI

Friendly AI

FOOM

Onomatopoetic vernacular for an intelligence explosion.

Fully general counterargument

An argument which can be used to discount any conclusion the arguer does not like.

Fuzzies

The desired but less useful counterpart to utils. They make you feel you're altruistic and socially contributing.

Hedon

A unit philosophers use to quantify pleasure. (Note: no actual quantifying is done.)

Hollywood rationality

What Spock does, not what actual rationalists do.

IA

Intelligence augmentation

IAWYC

I Agree With Your Conclusion

Generally used when nitpicking, to make it clear that the nitpicks are not meant to represent actual disagreement. Discussed in Support That Sounds Like Dissent.

I don't know

Something that can't be entirely true if you can even formulate a question.

IMO/IMHO

In my (humble) opinion

Inferential distance

The number of inferences, or intermediate steps, it takes someone to get from their existing knowledge to an understanding of the point you're making. See also illusion of transparency.

ISTM

It Seems To Me

Kolmogorov complexity

Given a string, the length of the shortest possible program that prints it.

LCPW

Least convenient possible world

A technique used to prevent oneself from evading the point of a question by nitpicking details.

Logical rudeness

A response to criticism which insulates the responder from having to address the criticism directly, without appearing to be conventional rudeness.

LW

Less Wrong

Meetup

Groups of Less Wrong members sometimes arrange to meet each other in meat space. Some geographic areas have groups that do this regularly.

Mind-killer

A topic that reliably produces biased discussions, e.g. politics or Pick-Up Artists.

MoR

Also HPMoR, Harry Potter and the Methods of Rationality

Motivated cognition

Reasoning used to reach desired conclusions rather than true conclusions.

MWI

Many-Worlds Interpretation, an interpretation of quantum mechanics advocated in Eliezer Yudkowsky's quantum mechanics sequence

NPC

Non-Player Character (think of an MMORPG like World of Warcraft that has characters controlled by humans and characters controlled by a computer; the characters controlled by humans would be PCs (player characters) and the characters controlled by a computer would be NPCs (non-player characters))

Noncentral fallacy

A rhetorical move often used in political, philosophical, and cultural arguments. "X is in a category whose archetypal member gives us a certain emotional reaction. Therefore, we should apply that emotional reaction to X, even though it is not a central category member."

OB

Overcoming Bias

One-box

One of the choices for Newcomb's problem.

Omega

A hypothetical superintelligent being, canonically found in Newcomb's problem.

Ontology/ontological

The philosophical study of the nature of being, existence, or reality, deals with questions concerning what entities exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences. See also ontological argument at Wikipedia for an example of (ab)using ontology to try and prove the existence of God.

Paperclip maximizer

An AI that has been created to maximize the number of paperclips in the universe. A hypothetical unfriendly artificial intelligence.

Paranoid debating

A group estimation game in which one player, unknown to the others, tries to subvert the group estimate.

Password

The answer you guess instead of actually understanding the problem. See Guessing the teacher's password

PC

Player Character (think of an MMORPG like World of Warcraft that has characters controlled by humans and characters controlled by a computer; the characters controlled by humans would be PCs (player characters) and the characters controlled by a computer would be NPCs (non-player characters))

PCT

Perceptual control theory

PD

Prisoner's dilemma

Philosophical zombie or P-Zombie

A creature which looks and behaves indistinguishably from a human down to the atomic level, but is not conscious. See Zombies (sequence)

Priors

What you update from in Bayesian calculations. In practical terms, everything you think you know now.

Privileging the hypothesis

The fallacy of singling out a specific hypothesis for investigation when there isn't enough evidence at hand to select this hypothesis over others.

"We have no idea who committed the murder, so let's consider the possibility that Mortimer Q. Snodgrass did it, and investigate him."

"The origin of the universe sure is mysterious! Have you considered that it could have been done by the God of the Bible?"

QALY

Quality-adjusted life year; a concept from the economics of health care

Rationalism

The mode of thinking within the rationalist movement

Rationalist taboo

A technique for unpacking words into concepts: taboo the use of a given word or its synonyms. Particularly useful in arguments over definitions.

Reversed stupidity is not intelligence

"The world's greatest fool may say the Sun is shining, but that doesn't make it dark out."

Reality is normal

Semantic stopsign

A term that looks like an explanation but, on closer examination, doesn't actually explain anything. Also called curiosity stopper.

Shut up and multiply

How to do a utility calculation without scope insensitivity.

Signaling

Conveying information by performing an action which would be costly to perform, if the information were not true.

Solomonoff induction

A formalized version of Occam's razor based on Kolmogorov complexity.

Steel man

A term for the opposite of a Straw Man: the strongest possible form of an opponent's argument, even if they didn't make it themself.

Strong man

Another term for the opposite of a Straw Man: the strongest actual form of an opponents argument.

Taboo the word ...

See rationalist taboo.

Teleology

Discussing an event as though it were caused by its future consequences.

tl;dr

Too long; didn't read.

Polite use: one-line summary at top of your long article. Impolite use: dismissive response to another's long piece of writing or unparagraphed slab of text.

Topic that must not be named

When LessWrong was started, Eliezer put a temporary moratorium on discussion of the Singularity or AI. You will see this used in old discussions to allude to these topics.

Two-box

One of the choices for Newcomb's problem.

Tsuyoku naritai

Japanese: "I want to become stronger."

uFAI

Unfriendly AI

Ugh field

A subject that is thought about less over time due to behavioral conditioning.

Utility function

A utility function assigns numerical values ("utilities") to outcomes, in such a way that outcomes with higher utilities are always preferred to outcomes with lower utilities.

Utils