The Collaborative International Dictionary of English v.0.48:

Anaesthetic \An`[ae]s*thet"ic\, Anesthetic \An`es*thet"ic\
   ([a^]n`[e^]s*th[e^]t"[i^]k), a. (Med.)
   (a) Capable of causing anesthesia; as, an[ae]sthetic agents.
   (b) Characterized by, or connected with, anesthesia; as, an
       an[ae]sthetic effect or operation.
       [1913 Webster + AS] anesthetic

The Collaborative International Dictionary of English v.0.48:

anaesthetic \an`[ae]s*thet"ic\, anesthetic \an`es*thet"ic\--AS,
   n. (Med.)
   A substance which produces anesthesia, as chloroform, ether,
   etc.
   [1913 Webster + AS]

WordNet (r) 3.0 (2006):

anaesthetic
    adj 1: relating to or producing insensibility [syn:
           anaesthetic, anesthetic]
    2: characterized by insensibility; "the young girls are in a
       state of possession--blind and deaf and anesthetic"; "an
       anesthetic state" [syn: anesthetic(a), anaesthetic(a)]
    n 1: a drug that causes temporary loss of bodily sensations
         [syn: anesthetic, anaesthetic, anesthetic agent,
         anaesthetic agent]