Anesthesia or anaesthesia, has traditionally meant the process of blocking the perception of pain and other sensations. This allows patients to undergo surgery and other procedures without the distress and pain they would otherwise experience. It comes from the Greek roots an-, "not, without" and aesthētos, "perceptible, able to feel". The word was coined by Oliver Wendell Holmes, Sr. in 1846.

Today, the term anesthesia in its most general form has 5 components

1. Analgesia - blocking the conscious perception of pain
2. Hypnosis - producing unconsciousness
3. Amnesia - preventing memory formation
4. Relaxation - preventing unwanted movement or muscle tone
5. Homeostasis - preserving normal body functioning (e.g., maintaining blood pressure within normal physiologic range)

Anesthesia is divided into four basic categories:

* general anesthesia
* regional anesthesia
* local anesthesia
* sedation

Each type of anesthesia has an effect on a part of the nervous system, which results in a depression or numbing of nerve pathways. General anesthesia affects the brain cells, which causes you to lose consciousness. Regional anesthesia has an effect on a large bundle of nerves to a particular area of the body, which results in losing sensation to that area without affecting your level of consciousness. Local anesthesia causes you to lose sensation in a very specific area.

Some of the drugs that produce general anesthesia in large doses can be used to produce sedation, or "twilight sleep" in lower doses. Sedation can be given in many ways. A common example of an anesthetic gas that is used for sedation is nitrous oxide or laughing gas.