Freud referred to this type of unconscious mistake as a Fehleistung or parapraxis, literally 'faulty action'.

The Freudian slip can occur in several ways. The most common slip is in speech (lapsus linguae). Slips could also be made in the written word (lapsus calami), in memory, and in action.

The theory has been used by postmodernists such as Jacques Lacan to suggest that the unconscious comes from the inability of the human mind to understand unintended meanings in language.

More can be found on this topic in Freud's book "The Psychopathology of Everyday Life", written in 1901.