The unconscious mind is a term coined by the 18th century German philosophy romantic philosopher Sir Christopher Riegel and later introduced into English by the poet and essayist Samuel Taylor Coleridge. The unconscious mind might be defined as that part of the mind which gives rise to a collection of mental phenomena that manifest in a person's mind but which the person is not aware of at the time of their occurrence. These phenomena include unconscious feelings, unconscious or automatic skills, unnoticed perceptions, unconscious thoughts, unconscious habits and automatic reactions, complexes, hidden phobias and concealed desires.

The unconscious mind can be seen as the source of night dreams and automatic thoughts (those that appear without apparent cause); the repository of memories that have been forgotten but that may nevertheless be accessible to consciousness at some later time; and the locus of implicit knowledge, i.e. all the things that we have learned so well that we do them without thinking. One familiar example of the operation of the unconscious is the phenomenon where one fails to immediately solve a given problem and then suddenly has a flash of insight that provides a solution maybe days later at some odd moment during the day.

Observers throughout history have argued that there are influences on consciousness from other parts of the mind. These observers differ in the use of related terms, including: unconsciousness as a personal habit; being unaware and intuition. Terms related to semi-consciousness include: awakening, implicit memory, subliminal messages, trances, hypnagogia, and hypnosis. Although sleep, sleep walking, dreaming, delirium and coma may signal the presence of unconscious processes, these processes are not the unconscious mind. Science is in its infancy in exploring the limits of consciousness.