The Collaborative International Dictionary of English v.0.48:

D'ebut \D['e]`but"\, n. [F. d['e]but, prop., the first cast or
   throw at play, fr. but aim, mark. See Butt an end.]
   A beginning or first attempt; hence, a first appearance
   before the public, as of an actor or public speaker. Debutant

WordNet (r) 3.0 (2006):

debut
    n 1: the act of beginning something new; "they looked forward to
         the debut of their new product line" [syn: introduction,
         debut, first appearance, launching, unveiling,
         entry]
    2: the presentation of a debutante in society
    v 1: present for the first time to the public; "The band debuts
         a new song or two each month"
    2: appear for the first time in public; "The new ballet that
       debuts next months at Covent Garden, is already sold out"
    3: make one's debut; "This young soprano debuts next month at
       the Metropolitan Opera"