The Collaborative International Dictionary of English v.0.48:

Ambitious \Am*bi"tious\, a. [L. ambitiosus: cf. F. ambitieux.
   See Ambition.]
   1. Possessing, or controlled by, ambition; greatly or
      inordinately desirous of power, honor, office,
      superiority, or distinction.
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            Yet Brutus says he was ambitious,
            And Brutus is an honorable man.       --Shak.
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   2. Strongly desirous; -- followed by of or the infinitive;
      as, ambitious to be or to do something.
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            I was not ambitious of seeing this ceremony.
                                                  --Evelyn.
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            Studious of song, and yet ambitious not to sing in
            vain.                                 --Cowper.
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   3. Springing from, characterized by, or indicating, ambition;
      showy; aspiring; as, an ambitious style.
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            A giant statue . . .
            Pushed by a wild and artless race,
            From off wide, ambitious base.        --Collins.
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WordNet (r) 3.0 (2006):

ambitious
    adj 1: having a strong desire for success or achievement [ant:
           ambitionless, unambitious]
    2: requiring full use of your abilities or resources; "ambitious
       schedule"; "performed the most challenging task without a
       mistake" [syn: ambitious, challenging]