The Collaborative International Dictionary of English v.0.48:

Sedative \Sed"a*tive\, a. [Cf. F. s['e]datif.]
   Tending to calm, moderate, or tranquilize; specifically
   (Med.), allaying irritability and irritation; assuaging pain.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sedative \Sed"a*tive\, n. (Med.)
   A remedy which allays irritability and irritation, and
   irritative activity or pain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sedative
    adj 1: tending to soothe or tranquilize; "valium has a
           tranquilizing effect"; "took a hot drink with sedative
           properties before going to bed" [syn: ataractic,
           ataraxic, sedative, tranquilizing,
           tranquillizing, tranquilising, tranquillising]
    n 1: a drug that reduces excitability and calms a person [syn:
         sedative, sedative drug, depressant, downer]