Piaget-....A stage then, we may say, is an integrated set of operational structures that constitute the thought processes of a person at a given time. Development involves the transformation of such " structures of the whole" in the direction of greater internal differentiation, complexity, flexibility and stability. A stage represents a kind of balanced relationship between a knowing subject and his or her environment. In this balanced or equilibrated position the person assimilates what is to be "known" in the environment into her or his existing structures of thought. When a novelty or challenge emerges that cannot be assimilated into the present structures of knowing then, if possible, the person accommmodates, that is , generates new structures of knowing. A stage transition has occured when enough accommodation has been undertaken to require ( and make possible) a transformation in the operational pattern of the structural whole of intellectual operations.
James W. Fowler, Stages of Faith: The Psychology of Human Development and the Quest for Meaning
development