Wordnet 3.0
ADJECTIVE (1)
1. 
 highly significant or important especially bringing about or marking the beginning of a new development or era; 
- Example: "epochal decisions made by Roosevelt and Churchill"- Example: "an epoch-making discovery"[syn: epochal, epoch-making]
WordNet (r) 3.0 (2006):
epoch-making
    adj 1: highly significant or important especially bringing about
           or marking the beginning of a new development or era;
           "epochal decisions made by Roosevelt and Churchill"; "an
           epoch-making discovery" [syn: epochal, epoch-making]