Moloch is the personification of the forces that coerce competing individuals to take actions which, although locally optimal, ultimately lead to situations where everyone is worse off. Moreover, no individual is able to unilaterally break out of the dynamic. The situation is a bad Nash equilibrium. A trap.

It happens when "In some competition optimizing for X, the opportunity arises to throw some other value under the bus for improved X. Those who take it prosper. Those who don’t take it die out. Eventually, everyone’s relative status is about the same as before, but everyone’s absolute status is worse than before. The process continues until all other values that can be traded off have been – in other words, until human ingenuity cannot possibly figure out a way to make things any worse." - Scott Alexander

One example of a Molochian dynamic is a Red Queen race between scientists who must continually spend more time writing grant applications just to keep up with their peers doing the same. Through unavoidable competition, they have all lost time while not ending up with any more grant money. And any scientist who unilaterally tried to not engage in the competition would soon be replaced by one who still does. If they all promised to cap their grant writing time, everyone would face an incentive to defect.

The topic of Moloch receives a formal treatment in the sequence Inadequate Equilibria, particularly in the chapter Moloch's Toolbox.

Origin

Scott Alexander linked the name to the concept in his eponymous post, Meditations on Moloch. The post intersperses lines of Allan Ginsberg's poem, Howl, with multiples examples of the dynamic including: the Prisoner's Dilemma, dollar auctions, fish farming story, Malthusian trap, capitalism, two-income trap, agriculture, arms races, races to the bottom, education system, science, and government corruption and corporate welfare.

From Allan Ginsberg's Howl:

What sphinx of cement and aluminum bashed open their skulls and ate up their brains and imagination?
Moloch! Solitude! Filth! Ugliness! Ashcans and unobtainable dollars! Children screaming under the stairways! Boys sobbing in armies! Old men weeping in the parks!
Moloch! Moloch! Nightmare of Moloch! Moloch the loveless! Mental Moloch! Moloch the heavy judger of men!
Moloch the incomprehensible prison! Moloch the crossbone soulless jailhouse and Congress of sorrows! Moloch whose buildings are judgment! Moloch the vast stone of war! Moloch the stunned governments!
Moloch whose mind is pure machinery! Moloch whose blood is running money! Moloch whose fingers are ten armies! Moloch whose breast is a cannibal dynamo! Moloch whose ear is a smoking tomb!
Moloch whose eyes are a thousand blind windows! Moloch whose skyscrapers stand in the long streets like endless Jehovahs! Moloch whose factories dream and croak in the fog! Moloch whose smoke-stacks and antennae crown the cities!
Moloch whose love is endless oil and stone! Moloch whose soul is electricity and banks! Moloch whose poverty is the specter of genius! Moloch whose fate is a cloud of sexless hydrogen! Moloch whose name is the Mind!

See also: Eldritch Analogies, Game Theory, Group Rationality, Social Reality