WordWeb Online

1. In a statistically significant way
   "the two groups differed significantly"
    
2. In a significant manner
   "our budget will be significantly affected by these new cuts"
    
3. In an important way or to an important degree
   "more significantly, Weber held that the manifold meaning attached to the event by the social scientist could alter his definition of the concrete event itself";
   - importantly
    
4. By a significant amount
   - greatly, markedly, muchly [informal]

Antonym: insignificantly