Luck is the phenomenon and belief that defines the experience of improbable events, especially improbably positive or negative ones. The naturalistic interpretation is that positive and negative events may happen at any time, both due to random and non-random natural and artificial processes, and that even improbable events can happen by random chance. In this view, the epithet "lucky" or "unlucky" is a descriptive label that refers to an event's positivity, negativity, or improbability.

Supernatural interpretations of luck consider it to be an attribute of a person or object, or the result of a favorable or unfavorable view of a deity upon a person. These interpretations often prescribe how luckiness or unluckiness can be obtained, such as by carrying a lucky charm or offering sacrifices or prayers to a deity. Saying someone is "born lucky" may hold different meanings, depending on the interpretation: it could simply mean that they have been born into a good family or circumstance; or that they habitually experience improbably positive events, due to some inherent property, or due to the lifelong favor of a god or goddess in a monotheistic or polytheistic religion.

Many superstitions are related to luck, though these are often specific to a given culture or set of related cultures, and sometimes contradictory. For example, lucky symbols include the number 7 in Christian-influenced cultures, the number 8 in Chinese-influenced cultures. Unlucky symbols and events include entering and leaving a house by different doors or breaking a mirror in Greek culture, throwing rocks into the wind in Navajo culture, and ravens in Western culture. Some of these associations may derive from related facts or desires. For example, in Western culture opening an umbrella indoors might be considered unlucky partly because it could poke someone in the eye, whereas shaking hands with a chimney sweep might be considered lucky partly because it is a kind but unpleasant thing to do given the dirty nature of their work. In Chinese and Japanese culture, the association of the number 4 as a homophone with the word for death may explain why it is considered unlucky. Extremely complicated and sometimes contradictory systems for prescribing auspicious and inauspicious times and arrangements of things have been devised, for example feng shui in Chinese culture and systems of astrology in various cultures around the world.

Many polytheistic religions have specific gods or goddesses that are associated with luck, both good and bad, including Fortuna and Felicitas in the Ancient Roman religion (the former related to the words "fortunate" and "unfortunate" in English), Dedun in Nubian religion, the Seven Lucky Gods in Japanese mythology, mythical American serviceman John Frum in Polynesian cargo cults, and the inauspicious Alakshmi in Hinduism.