What Do We Mean By ‘Rationality’? (as linked below by saturn) is a good place to start, and it makes an important point: “We are not here to argue the meaning of a word, not even if that word is ‘rationality’.” (Or “rationalism”.) That page describes the basic things-we’re-trying-to-do on Less Wrong, and “rationality” is the shorthand we use to refer to those things; roughly speaking, a “rationalist” is a person who aspires to improve both their epistemic rationality and instrumental rationality (as described on that page), particularly by improving their awareness of the threats posed to their rationality by things that human brains do by default, such as the many cognitive biases and fallacies.
(And nobody here will claim that that’s the One True Meaning of “rationalism”; again, with the conceptual associations it has here, it should be taken mostly as local shorthand. Many of us would argue that it’s among the best of all the various things people have called “rationalism”, but we know quite well that this is a claim that cannot be supported just by giving a definition.)
Off the top of my head, a few more interesting discussions on the subject of rationality as an “ism”:
What Do We Mean By ‘Rationality’? (as linked below by saturn) is a good place to start, and it makes an important point: “We are not here to argue the meaning of a word, not even if that word is ‘rationality’.” (Or “rationalism”.) That page describes the basic things-we’re-trying-to-do on Less Wrong, and “rationality” is the shorthand we use to refer to those things; roughly speaking, a “rationalist” is a person who aspires to improve both their epistemic rationality and instrumental rationality (as described on that page), particularly by improving their awareness of the threats posed to their rationality by things that human brains do by default, such as the many cognitive biases and fallacies.
(And nobody here will claim that that’s the One True Meaning of “rationalism”; again, with the conceptual associations it has here, it should be taken mostly as local shorthand. Many of us would argue that it’s among the best of all the various things people have called “rationalism”, but we know quite well that this is a claim that cannot be supported just by giving a definition.)
Off the top of my head, a few more interesting discussions on the subject of rationality as an “ism”:
Rationalists should beware rationalism
A rational identity
Rationality is systematized winning