Anesthesia, or anaesthesia (see spelling differences; from Greek lang: αν-, an-, "without"; and lang: αἲσθησις, aisthēsis, "sensation"), has traditionally meant the condition of having sensation (including the feeling of pain) blocked or temporarily taken away. This allows patients to undergo surgery and other procedures without the distress and pain they would otherwise experience. The word was coined by Oliver Wendell Holmes, Sr. in 1846. Another definition is a "reversible lack of awareness", whether this is a total lack of awareness (e.g. a general anaesthestic) or a lack of awareness of a part of the body such as a spinal anaesthetic or another nerve block would cause. Anesthesia differs from analgesia in blocking all sensation, not only pain.

Today, the term general anesthesia in its most general form can include:

• Analgesia: blocking the conscious sensation of pain;
• Hypnosis: producing unconsciousness;
• Amnesia: preventing memory formation;
• Paralysis: preventing unwanted movement or muscle tone;
• Obtundation of reflexes, preventing exaggerated autonomic reflexes.

Patients undergoing anesthesia usually undergo preoperative evaluation. It includes gathering history of previous anesthetics, and any other medical problems, physical examination, ordering required blood work and consultations prior to surgery.

There are several forms of anesthesia. The following forms refer to states achieved by anesthetics working on the brain:

• General anesthesia: "Drug-induced loss of consciousness during which patients are not arousable, even by painful stimulation." Patients undergoing general anesthesia can often neither maintain their own airway nor breathe on their own. While usually administered with inhalational agents, general anesthesia can be achieved with intravenous agents, such as propofol.
• Deep sedation/analgesia: "Drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully following repeated or painful stimulation." Patients may sometimes be unable to maintain their airway and breathe on their own.
• Moderate sedation/analgesia or conscious sedation: "Drug-induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation." In this state, patients can breathe on their own and need no help maintaining an airway.
• Minimal sedation or anxiolysis: "Drug-induced state during which patients respond normally to verbal commands." Though concentration, memory, and coordination may be impaired, patients need no help breathing or maintaining an airway.

The level of anesthesia achieved ranges on a continuum of depth of consciousness from minimal sedation to general anesthesia. The depth of consciousness of a patient may change from one minute to the next.