Anesthesia, or anaesthesia (see spelling differences; from Greek αν-, an-, "without"; and 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.