What a man can be, he must be.Abraham Maslow — Motivation and Personality, 1954
Self-actualization is Abraham Maslow's term for the full expression of distinctively human capacities — the actualization of one's potential, the becoming of what one genuinely is. It is not happiness in the hedonic sense. It is the experience of functioning at the full depth of one's nature: creative, engaged, present, purposeful, and in genuine contact with both oneself and the world.
Maslow placed it at the apex of his hierarchy of needs — above safety, belonging, and esteem — on the grounds that it can only be sought when more fundamental needs are sufficiently met. But he was careful to note that this does not make it optional. The person who has the capacity for self-actualization and fails to develop it is not simply missing a luxury. They are in a state of what he called meta-pathology: not clinically ill, but diminished, unfulfilled, chronically below what they could be.
The concept is older than Maslow's term for it. Aristotle called it eudaimonia — the full exercise of distinctively human capacities in accordance with virtue. Vivekananda taught that each soul is potentially divine and that the goal of all human practice is to manifest that divinity. Emerson said that envy is ignorance and imitation is suicide — that what you alone can contribute to existence cannot be contributed by anyone else, and that failing to contribute it is a form of waste. All of these are versions of the same claim: human beings are not finished when they are born, and the work of becoming fully human is the most important work there is.
Self-actualization is not a destination. Maslow found that self-actualizing people experienced what he called peak experiences — brief moments of complete absorption, profound meaning, and unity — but that they then returned to ordinary life, more themselves than before. The movement is not toward a fixed end state but toward an ongoing process of becoming.