Bad faith (from French mauvaise foi) is a philosophical concept used by existentialist philosophers Jean-Paul Sartre and Simone de Beauvoir to describe the phenomenon where human beings under pressure from social forces adopt false values and disown their innate freedom, hence acting inauthentically. It is closely related to the concepts of self-deception and ressentiment.
A critical claim in existentialist thought is that individuals are always free to make choices and guide their lives towards their own chosen goal or "project". The claim holds that individuals cannot escape this freedom, even in overwhelming circumstances. For instance, even an empire's colonized victims possess choices: to submit to rule, to negotiate, to commit suicide, to resist nonviolently, or to counter-attack.
Although external circumstances may limit individuals (this limitation from the outside is called facticity), they cannot force a person to follow one of the remaining courses over another. In this sense the individual still has some freedom of choice. For this reason, individuals choose in anguish: they know that they must make a choice, and that it will have consequences. For Sartre, to claim that one amongst many conscious possibilities takes undeniable precedence (for instance, "I cannot risk my life, because I must support my family") is to assume the role of an object in the world, not a free agent, but merely at the mercy of circumstance (a being-in-itself that is only its own facticity, i.e., it "is" inside itself, and acts there as a limitation).
For Sartre this attitude is manifestly self-deceiving. As conscious humans, we are always aware that we are more than what we are aware of, so we are not whatever we are aware of. We cannot, in this sense, be defined as our "intentional objects" of consciousness, including our restrictions imposed by (facticity) our personal history, character, bodies, or objective responsibility. Thus, as Sartre often repeated, "Human reality is what it is not, and it is not what it is." An example would be if one were now a doctor but wished and started to "transcend" to become a pig farmer. One is who one is not (a pig farmer), not who one is (a doctor): it can only define itself negatively, as "what it is not"; but this negation is simultaneously the only positive definition it can make of "what it is."