Rationality is the ability to reason about the environment, and to plan actions to manipulate it, usually with logic. There is a difficulty here in defining rationality objectively (by behavior), or subjectively (by what is happening internally of the mind of the subject).
Rationality is not a goal, it is merely a tool. Given possible actions of an individual, the more rational action is the action which maximises its utility. However utility itself is not possible to define for most intelligent individuals (though it is possible to observed through expressed preference if the individual is given free will).
Rationality is the essence of humanity.