I would like to suggest that we try to come up with several defintions of rationality. I don't feel we have exhausted this search area by any means. Robin has suggested, "More "rational" means better believing what is true, given one's limited info and analysis resources". Other commenters have emphasised goal-directed behaviour as a necessary ingredience of rationality. I think these defintions miss out on several important ingrediences - such as the social nature of rationality. There is also a subtext which argues - that rationality only gives one (correct) answer even if we only can approximate it. I feel strongly that rationality can give several correct answers and thus imagination is an ingredience of rationality. So without in any way believing that I have found the one correct defintion, I propose the following: When two or more brains try to be sensible about things and expand their agency. I believe that "sensible" in this context does not need to be defined as it is a primitive and each player willl submit their own meaning.
Maybe this is a can of worms - but are there other suggestions or defintions for rationality we can apply in our lives?