I just found this:

https://en.wikibooks.org/wiki/Princi...nomics/Utility

I recommend reading the whole thing (it's short), but here's the part about rationality (bold is mine):

In economics, we usually say that an individual is "rational" if that individual maximizes utility in their decisions. That is, whenever an individual is to choose between a group of options, they are rational if they choose the option that, all else equal, gives the greatest utility. Recalling that utility includes every element of a decision, this assumption is not particularly difficult to accept. If, when everything is taken into account, one decision provides the greatest utility, which is equivalent to meaning that it is the most preferred, then we would expect the individual to take that most preferred option.

This should not necessarily be taken to mean that individuals who fail to quantify and measure every decision they make are behaving irrationally. Rather, this means that a rational individual is one who always selects that option that they prefer the most.
The rationality assumption may seem trivial, but it is basic to the study of economics. This assumption gives a basis for modeling human behavior and decision making. If we could not assume rationality, it would be impossible to say what, when presented with a set of choices, an individual would select. The notion of rationality is therefore central to any understanding of microeconomics.