The shortest possible way we can define Bildung is this: knowing your roots and being able to imagine the future.
The word Bildung does not exist in English and we therefore use the German word, which entails many different aspects of the formation and transformation of our inner world.
Bildung is always Bildung in a societal and cultural context. What is useful knowledge and good behavior in one culture may be useless or offensive in another. What is universal across cultures is that individuals go through personal development towards increasing knowledge, consciousness, emotional freedom, autonomy, and sense of responsibility.
One aspect of Bildung could be called inner world expansion: the accumulation of deliberate education, enculturation, general life-experience and moral and emotional development. All four, separately as well as together, are ongoing processes and continue to evolve as we encounter the world. They are also interdependent, and together, these aspects of our cognitive, intellectual and emotional development produce a multi-dimensional inner world that can be stimulated, inspired and challenged in many different ways.
Another aspect relates to existential meaning and autonomy: In order for individuals to be empowered and gain autonomy over their own life, part of the moral and emotional development is existential meaning, a sense of purpose and personal agency.
Both the expansion of the inner world and the existential meaning and autonomy evolve and emerge from engaging with the surrounding culture, which provides us with language, aesthetics, symbolic worlds, moral values, and ethics and thus the ability to connect with others and with society.
Bildung, therefore, is a process of individual maturing and taking upon oneself ever bigger personal responsibilities towards family, friends, fellow citizens, society, humanity, our globe, and the global heritage of our species, while enjoying ever greater personal, moral and existential freedoms. Bildung is the enculturation and life-long learning that allows us to grow and change, it is existential and emotional depth, it is life-long interaction and struggles with new knowledge, culture, art, science, morality, ethics, spirituality, religion, aesthetics, perspectives, people, and truths, and it is being an active citizen in adulthood. Bildung is emancipation and a constant process that never ends.
The process in this development can be happenstance or a deliberate choice and any mix thereof, and Bildung can be summed up, not as a curriculum, but as CREATE: Consciousness Responsibility Empowerment Action Transformation Emancipation.