Doubt is uncertainty in the context of trust (where it takes the form of distrust), action, decision or faith. It implies challenging some notion of truth in effect, or hesitating to take action on it due to concern that one might be mistaken.

According to some spiritual and ethical traditions, it is a form of fear. According to other traditions, it is wholly rational and causes us to hesitate before acting, and apply more rigorous methods.

The scientific method, and to a degree all of science can be said to be entirely motivated by doubt: rather than accept the existing theories, experiments to test them continue. Technology can be seen as simply the expansion of the experiments to a wider user base, who take real risks with it. Users may no longer doubt the applicability of the theory in play, but there remain doubts about how it interacts with the real world. The process of technology transfer stages exploitation of science to ensure that doubt and danger are minimized.

Doubt is very often debated in the context of Christianity where it refers to doubt about salvation and eventual redemption in an afterlife. This issue has become particularly important in the Protestant version of this faith, where only acceptance of Jesus as a saviour and intermediary with God is required for a positive outcome. The debate is less important in most other religions and ethical traditions.

Doubt is the only human emotion which proves its own existence as "I doubt that (the concept of) doubt exists, therefore it exists".

In politics, ethics and law, where very important decisions are made that often determine the course of someone's life, doubt is central, and often motivates an elaborate adversarial process to carefully sort through all the evidence to come to a decision.

A 2003 book by Jennifer Michael Hecht, titled "Doubt: A History", traces the role of doubt throughout time, all over the world, particularly regarding religion.

Doubt (play) is also the name of a 2004 play by John Patrick Shanley.

See also: FUD, skepticism, Doubting Thomas, list of ethics topics