WordWeb Online

1. Of extreme importance; vital to the resolution of a crisis
   "a crucial moment in his career";
   - important
    
2. Having essential relevance
   "crucial to the case"
    
3. Of the greatest importance
   "crucial information";
   - all-important, all important, essential, of the essence
    
4. [slang] Brilliant; very good
   - socko [N. Amer, informal], righteous [US, slang], fab [Brit, informal], rad [N. Amer, slang], radical [N. Amer, slang], nang [Brit, slang], phat [slang], legit [US, slang], awesomesauce [informal], amazeballs [informal], beaut [Austral, NZ, informal], lit [US, slang], storming [Brit, informal], blinding [Brit, slang], stonking [Brit, informal], brill [Brit, informal], wicked [slang], cushty [Brit, informal], ace [informal], def [slang], magic [Brit, slang], smokin' [slang], smoking [slang], super [informal], sick [N. Amer, slang]

See also: A1 [informal], ace [informal], A-one [informal], awesome [informal], beezer [UK, archaic], brilliant, climacteric, crack [informal], critical, decisive, essential, excellent, fantabulous [informal], first-class, first-rate, imperative, important, life-and-death, life-or-death, lush [Brit, informal], material, mean [informal], of import, outstanding, paramount, pivotal, polar, ripping [Brit, informal], splendid, stellar [informal], super, tiptop, top-drawer, topnotch [informal], top-notch [informal], tops [informal]

Encyclopedia: Crucial