noun

• (medicine, American spelling, Canadian spelling) An artificial method of preventing sensation, used to eliminate pain without causing loss of vital functions, by the administration of one or more agents which block pain impulses before transmitted to the brain.
  "In addition, 0.5 ml of 1% lidocaine HCl was injected subcutaneously in the postauricular area for local anesthesia."
• (American spelling) The loss or prevention of sensation, as caused by anesthesia (in the above sense), or by a lesion in the nervous system, or by another physical abnormality.
  "In some individuals optimism may become quasi-pathological. The capacity for even a transient sadness or a momentary humility seems cut off from them as by a kind of congenital anæsthesia."
• (loosely, metonymic, often proscribed) A medication that provides the service of temporarily blocking sensation.