The concept enters philosophy through Kierkegaard, who was concerned less with self-expression than with the difference between beliefs you have genuinely made your own and beliefs you hold because everyone around you does. The second kind can be perfectly sincere and still not yours in the sense that matters — you would have held the opposite had you been born elsewhere.
Heidegger gave this a structural account. Much of the time we exist in a mode he called das Man — usually translated as "the they" — in which we do what one does, want what one wants, and find interesting what one finds interesting. This is not a moral failing but the default condition, and it is genuinely comfortable: the anonymous "one" absolves you of having to decide. Authenticity is not escaping society but taking ownership of an existence that was being lived on your behalf.
Sartre's contribution is the sharpest and the most usable. Bad faith is the self-deception of denying your own freedom — treating a choice as a necessity. The waiter who performs his role so completely that he becomes a thing rather than a person choosing to do a job; the person who says "I had no option" about something they did in fact choose; the person who treats their character as fixed to avoid the work of changing it. What makes bad faith paradoxical is that you must know the truth in order to hide it from yourself, which is why Sartre calls it a lie to oneself rather than simple ignorance.
Notice that none of these accounts says "discover who you really are and express it." Sartre's position rules that out explicitly: since existence precedes essence, there is no fixed nature to uncover. Authenticity is not fidelity to a hidden self but honest acknowledgement that you are choosing, and that the choosing is yours.
ObjectionsThe difficulties.
True to what? The obvious problem. If there is no essential self, authenticity seems to lose its object. The existentialist answer — that you are being true to your freedom rather than to an essence — is coherent, but it makes authenticity a formal condition rather than a guide to what to do, which is less than the concept is usually asked to deliver.
It can excuse anything. Someone can be cruel authentically. If authenticity is purely about owning your choices, it says nothing about whether the choices are good, and it can dress selfishness as integrity. De Beauvoir saw this and argued that authentic freedom commits you to willing others' freedom too — the movement's best attempt at closing the gap.
The social objection. Charles Taylor has argued that the popular version degrades into self-absorption precisely by forgetting that our identities are formed in dialogue with others. On this view authenticity is not opposed to social life; it is the demand that your participation in it be genuinely yours rather than automatic.