/
anˈtɪθəsɪs/
n.
 (pl. antitheses  
/-siːz/
)
   - a person or thing that is the direct opposite of another. ▸a contrast or opposition between two things. 
- a rhetorical or literary device in which an opposition or contrast of ideas is expressed.
- (in Hegelian philosophy) the negation of the thesis as the second stage in the process of dialectical reasoning. Compare with synthesis.
Etymology
ME: from late L., from Gk antitithenai ‘set against’.