The Collaborative International Dictionary of English v.0.48:

Valedictorian \Val`e*dic*to"ri*an\, n.
   One who pronounces a valedictory address; especially, in
   American colleges, the student who pronounces the valedictory
   of the graduating class at the annual commencement, usually
   the student who ranks first in scholarship.
   [1913 Webster]

WordNet (r) 3.0 (2006):

valedictorian
    n 1: the student with the best grades who usually delivers the
         valedictory address at commencement [syn: valedictorian,
         valedictory speaker]