In the practice of medicine (especially surgery) and dentistry, anesthesia or anaesthesia is a temporary induced state with one or more of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), amnesia (loss of memory), and unconsciousness. A patient under the effects of anesthetic drugs is referred to as being anesthetized.

Anesthesia enables the painless performance of medical procedures that would cause severe or intolerable pain to an unanesthetized patient. Three broad categories of anaesthesia exist:

• General anesthesia suppresses central nervous system activity and results in unconsciousness and total lack of sensation.
• Sedation suppresses the central nervous system to a lesser degree, inhibiting both anxiety and creation of long-term memories without resulting in unconsciousness.
• Regional anesthesia and local anesthesia, which block transmission of nerve impulses between a targeted part of the body and the central nervous system, causing loss of sensation in the targeted body part. A patient under regional or local anesthesia remains conscious. Two broad classes exist:
• Peripheral blockade inhibits sensory perception in an isolated part of the body, such as numbing a tooth for dental work or administering a nerve block to inhibit sensation in an entire limb.
• Central, or neuraxial, blockade administers the anesthetic in the region of the central nervous system itself, suppressing incoming sensation from outside the area of the block. Examples include epidural anaesthesia and spinal anaesthesia.
• In preparing for a medical procedure, the health care provider giving anesthesia chooses and determines the doses of one or more drugs to achieve the types and degree of anesthesia characteristics appropriate for the type of procedure and the particular patient. The types of drugs used include general anesthetics, hypnotics, sedatives, neuromuscular-blocking drugs, narcotic, and analgesics.