Anesthesia or anaesthesia (see spelling differences; from Greek αν- an- “without” + αἲσθησις aisthesis “sensation”) has traditionally meant the condition of having the feeling of pain and other sensations blocked. 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 a the body such as a spinal anaesthetic or another nerve block would cause.

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

• Analgesia: blocking the conscious sensation of pain;
• Hypnosis: produces unconsciousness without analgesia;
• Amnesia: preventing memory formation;
• Relaxation: preventing unwanted movement or muscle tone;
• Obtundation of reflexes, preventing exaggerated autonomic reflexes.

Patients undergoing surgery 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.

The following refer to states achieved by anesthetics working outside of the brain:

• Regional anesthesia: Loss of pain sensation, with varying degrees of muscle relaxation, in certain regions of the body. Administered with local anesthesia to peripheral nerve bundles, such as the brachial plexus in the neck. Examples include the interscalene block for shoulder surgery, axillary block for wrist surgery, and femoral nerve block for leg surgery. While traditionally administered as a single injection, newer techniques involve placement of indwelling catheters for continuous or intermittent administration of local anesthetics.
• Spinal anesthesia: also known as subarachnoid block. Refers to a Regional block resulting from a small volume of local anesthetics being injected into the spinal canal. The spinal canal is covered by the dura mater, through which the spinal needle enters. The spinal canal contains cerebrospinal fluid and the spinal cord. The sub arachnoid block is usually injected between the 4th and 5th lumbar vertebrae, because the spinal cord usually stops at the 1st lumbar vertebra, while the canal continues to the sacral vertebrae. It results in a loss of pain sensation and muscle strength, usually up to the level of the chest (nipple line or 4th thoracic dermatome).
• Epidural anesthesia: Regional block resulting from an injection of a large volume of local anesthetic into the epidural space. The epidural space is a potential space that lies underneath the ligamenta flava, and outside the dura mater (outside layer of the spinal canal). This is basically an injection around the spinal canal.
• Local anesthesia is similar to regional anesthesia, but exerts its effect on a smaller area of the body.