valediction, valedictory address, valedictory oration, valedictory (adj)
a farewell oration (especially one delivered during graduation exercises by an outstanding member of a graduating class)
of or relating to an occasion or expression of farewell
"a valedictory address"; "valedictory praise for his uniformly manly course"; "a suitable valedictory gesture"
of a speech expressing leave-taking
"a valedictory address"
valedictorian, valedictory speaker (noun)
the student with the best grades who usually delivers the valedictory address at commencement