The Collaborative International Dictionary of English v.0.48:

Vocative \Voc"a*tive\, a. [L. vocativus, fr. vocare to call.]
   Of or pertaining to calling; used in calling; specifically
   (Gram.), used in address; appellative; -- said of that case
   or form of the noun, pronoun, or adjective, in which a person
   or thing is addressed; as, Domine, O Lord.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vocative \Voc"a*tive\, n. [L. vocativus (sc. casus): cf. F.
   vocatif.] (Gram.)
   The vocative case.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vocative
    adj 1: relating to a case used in some languages; "vocative verb
           endings"
    n 1: the case (in some inflected languages) used when the
         referent of the noun is being addressed [syn: vocative,
         vocative case]