Outside of LessWrong, "humility" usually refers to "a modest or low view of one's own importance". In common parlance, to be humble is to be meek, deferential, submissive, or unpretentious, "not arrogant or prideful". Thus, in ordinary English "humility" and "modesty" have pretty similar connotations.

On LessWrong, Eliezer Yudkowsky has proposed that we instead draw a sharp distinction between two kinds of "humility" — social modesty, versus "scientific humility".

In The Proper Use of Humility (2006), Yudkowsky writes:

You suggest studying harder, and the student replies: “No, it wouldn’t work for me; I’m not one of the smart kids like you; nay, one so lowly as myself can hope for no better lot.”
This is social modesty, not humility. It has to do with regulating status in the tribe, rather than scientific process.
If you ask someone to “be more humble,” by default they’ll associate the words to social modesty—which is an intuitive, everyday, ancestrally relevant concept. Scientific humility is a more recent and rarefied invention, and it is not inherently social. Scientific humility is something you would practice even if you were alone in a spacesuit, light years from Earth with no one watching. Or even if you received an absolute guarantee that no one would ever criticize you again, no matter what you said or thought of yourself. You’d still double-check your calculations if you were wise.

On LW, then, we tend to follow the convention of using "humility" as a term of art for an important part of reasoning: combating overconfidence, recognizing and improving on your weaknesses, anticipating and preparing for likely errors you'll make, etc.

In contrast, "modesty" here refers to the bad habit of letting your behavior and epistemics be ruled by not wanting to look arrogant or conceited. Yudkowsky argues in Inadequate Equilibria (2017) that psychological impulses like "status regulation and anxious underconfidence" have caused many people in the effective altruism and rationality communities to adopt a "modest epistemology" that involves rationalizing various false world-models and invalid reasoning heuristics.

LW tries to create a social environment where social reward and punishment is generally less salient, and where (to the extent it persists) it incentivizes honesty and truth-seeking as much as possible. LW doesn't always succeed in this goal, but this is nonethe...