Nurse Ratched (also known as "Big Nurse") is a fictional character and the main antagonist of Ken Kesey's 1962 novel One Flew Over the Cuckoo's Nest, as well as the 1975 film. A cold, heartless tyrant, Nurse Ratched has become the stereotype of the nurse as a battleaxe. She has also become a popular metaphor for the corrupting influence of power and authority in bureaucracies such as the mental institution in which the novel is set.
In the novel 
Nurse Ratched is the head administrative nurse at the Salem, Oregon State Hospital, a mental institution where she exercises near-absolute power over the patients' access to medications, privileges, and basic necessities such as food and toiletries. She capriciously revokes these privileges whenever a patient displeases her. Her superiors turn blind eyes because she maintains order, keeping the patients from acting out, either through antipsychotic and anticonvulsant drugs or her own brand of "therapy", which consists mostly of humiliating patients into doing her bidding. Her greatest success is the stuttering, suicidal Billy Bibbit, who is so terrified of her that he does whatever she says.
When Randle McMurphy arrives at the hospital, however, her dictatorial rule is nearly toppled; he flouts her rules with impunity, and inspires other patients to follow. Her attempts to cow him into submission — at first with threats and mild punishments, then with shock therapy — are unsuccessful, serving only to fuel his defiance.
Eventually McMurphy sneaks his prostitute girlfriend into the asylum, and encourages her to relieve Billy of his virginity. Ratched threatens to tell Billy's mother about the transgression. Since Billy fears his mother as much as or more than he fears Ratched, the threat frightens him into committing suicide. Enraged, McMurphy attacks Ratched, and nearly chokes her to death.
In retribution, Ratched has McMurphy lobotomized. Chief Bromden, another patient and the narrator of the book, later smothers McMurphy as a mercy killing. However, due to McMurphy's permanent damage to her vocal cords, Ratched's cruel voice has been removed, rendering her unable to speak well, and requiring her to often resort to writing notes to communicate, which diminishes her control over the other patients.
Differences between the novel and the film 
Nurse Ratched's first name is revealed to be Mildred in the film version. Also, after being strangled, Ratched is shown injured in a neck brace but, unlike in the novel, is able to speak in a thin, reedy voice.
Louise Fletcher won the Academy Award for Best Actress for her portrayal of Nurse Ratched in the film. Nurse Ratched was named the fifth-greatest villain in film history by the American Film Institute in their series 100 Years... 100 Heroes & Villains.