I like the word "optimal", and it frequently overlaps with "rational", but they aren't interchangeable. I think the key difference is that the word "rationality" carries along an implicit human actor with beliefs, knowledge and goals. On the other hand, the word "optimal" frequently ends up pointed at narrow, abstract goals, omniscient viewpoints and unavailable options.

"Investing in ABCD turned out to be suboptimal, but it was rational because he couldn't have known their factory would be hit by an earthquake." (Optimality is with respect to a state of perfect information, but past-tense rationality is with respect to the information that was actually available.)
"Sending a letter was suboptimal, but it was rational because I didn't have an email address." (Optimization is over an implied set of contact methods, rationality counts only those that are available.) "That algorithm is optimal, but using it would be irrational because it's complicated. (Implied narrow goal: computational efficiency. Actual goal system includes simple implementation.)

Since talking about rationality frequently means talking about how to deal with limited information and clarifying confusions about goals, the word "optimal" doesn't always work. There's another important difference, too, which is that "rationality" is more meta; and this protects it somewhat, in that I'd expect a book about "optimal decision-making" to tell me what to do, and a book about "rational decision-making" to tell me how to decide. The latter is more trustworthy, since the layer of indirection makes it hard to slip in agendas.

When I first heard that Lesswrong was full of rationalists, I assumed that Lesswrong was anti-empiricist.

What's the short description-word alternative then? "Rationality" and "aspiring rationalist" sounds better than "optimized behavior" or anything else I see along these lines; surely one's behavior is never already "optimal". How would you call a practitioner? "Student of optimization"? "Optimized one"? This is way more weird than "rationalist".

I'm going to continue to say optimized when I mean optimized and continue to say rational when I mean rational. I'm also going to continue to refrain from making titles along the lines of "rational shoe buying" - except when I've accidentally drunk something poisonous and don't have a stomach pump available.

I think rational should be reserved for some level of conscious choice, probably conceptually mediated.

I don't want to say you're more rational than I am if your enzymes are more efficient, or if you react more effectively to avoid the rush of oncoming defensive linemen than I do. Optimal is much broader than rational. By rational we mean consciously choosing the optimal, not just being optimal.

Of course, status quo bias being what it is, I promptly added that we wouldn't actually want to do that, because, y'know, it would be work and involve changing stuff.

(Taking the difficulty of changing things into account is not necessarily status quo bias. Sometimes it's really better to live with imperfections of status quo than to pay the cost of the change. This isn't necessarily relevant here though.)

I think superlatives like "optimal" have a lot of problems. In particular, I think saying "optimal philanthropy" says to people "I'm perfect, unlike you," while comparatives like "better philanthropy" communicate something more like "Let us strive together to be better." (I think "efficient" is a better choice than "better" for related connotative reasons.)

As a bonus, "optimalism" isn't already taken (unlike rationalism). We certainly don't want to be confused with optimists though.

That is a very good point here. A stupid intelligence system (the one that explores too little of the solution space) can do things that are grossly suboptimal, but are nonetheless rational in the narrow sense that the system did take the effort to try to explore solution space and to pick best solution. Humans though tend to choose what parts of solution space to explore based on all sorts of biases, many of which are outright irrational, in the sense of not being backed with correct reasoning, or being backed with deliberately faulty reasoning as a cover story.

A new arrival, Kouran, recently challenged our conventional use of the label "rational" to describe various systems. The full thread is here, and it doesn't summarize neatly, but he observes that we often use "rational" in the context of non-intellectual, non-cognitive, etc. systems, and that this is an unconventional use of the word.

I read the comment by Kouran and while he or she had some valid points they are for most part already points assumed by our actual conventions here and so do not particularly apply as criticism. Mind you this is from a perspective of someone already inclined to consider some usages of 'rational' as a violation of convention.

Reread What Do We Mean By "Rationality".

"X is rational!" is usually just a more strident way of saying "I think X is true" or "I think X is good". So why have an additional word for "rational" as well as "true" and "good"? Because we want to talk about systematic methods for obtaining truth and winning.

(emphasis in original)

I think the same goes for 'optimal'. When we say we're studying the art and science of human rationality, we don't mean we're studying what's optimal for humans. It means we're studying how humans can go about achieving optimality, using systematic, repeatable methods.

I've been using the label "LessWrongian" on occasion with friends. I've also been wondering recently if just plain "wisdom" would be a more catch-all label for the type of rationality we're talking about.

I assume you mean formulations (especially post titles) like "rational home buying", "rational childcare" or "rational dating".

While I do not like those formulations, I like formulations that replace "rational" with "optimal" even less. To me the word "optimal" is mostly a mathematical term (Wikipedia redirects optimal to mathematical optimization) and does not acknowledge that humans have complex values, limited knowledge and resources and that there are costs in those activities. Also I understand "optimal" to mean maximally good. If something could be better, it's not yet optimal. That means something like optimal childcare might require super human capabilities.

Formulations like "rational home buying" sound silly and even cultish, because "rational" is completely redundant here (it makes you wonder why they won't shut up about their favorite subject "rationality"). "Rational home buying" is just about home buying. No one is is deliberately irrational. I'd suggest to just say "home buying", unless you really want to contrast it with irrational home buying.

I usually use "rational" in a somewhat jargonish sense to refer to things that have goals and work to achieve them in a broad-context way. This is pretty dang distinct from "optimal," mainly because calling something optimal implies something to optimize, while goals can be anything they want.

Do you mean the use of "rational" as in the "rational shoe buying" threads? In that case, sure.

Prior to LessWrong, I always remembered rational as meaning "free of logical fallacy". A perfectly rational person was someone who, when given a group of premises, would always derive every logically valid conclusion possible from that set, and never derive a logically invalid one.