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Synonyms for Ambitious (same or very similar meaning)

WordNet sense 1 (requiring full use of your abilities or resources):

WordNet sense 2 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):
difficult, hard

WordNet sense 3 (strongly motivated to succeed):

WordNet sense 4 (desiring or striving for recognition or advancement):

WordNet sense 5 (marked by aggressive ambition and energy and initiative):
pushing, pushy, pushful

From the ODE community, based on WordNetadd/edit