There is no unique and definitive definition of phenomenology. It is rather a method and an experience always open and always renewing itself. Phenomenology involves a change in the "sense of the world": everything acquires its sense and value only when it becomes the content of the lived experience of the subject correlated to his intentional acts. This is the main thesis of the phenomenological method aiming at overcoming the traditional opposition between rationalism and empiricism.
Starting from Husserl, the father of this approach, the history of phenomenology undertook different and unexpected developments which in some cases were rather far away from Husserl's original thought. In the U.K. attention has been given to an analytical-epistemological phenomenology focused on the relationship between intentionality and logical semantics. In France it is mainly an anthropological-existential phenomenology. In Germany an hermeneutic phenomenology was developed, mainly by Heidegger and Gadamer.
Regardless of these raw distinctions, a big question is so far unresolved: how to reconcile the phenomenological/ existential stance claiming for the irreducibility of each lived experience and the scientific paradigm? Is it possible to imagine brain mechanisms and physiological systems explaining the endless mysteries and manifold paradoxes of the human being?
Phenomenology claims that a human being can never be considered as an object, as if he was a natural thing; rather the task is to understand him as the focus of a relationship linking subjective attitudes to the objects showed by the experience. In this sense, an important contribution was Merleau-Ponty's view that man is not something psychic joined to an organism, but a sort of fluctuation of the existence that sometimes is a bodily one, sometimes refers to personal acts. Consequently, he proposes to reinstate in the existence both its "physiological" and "psychic" sides both being intentionally oriented towards a world.