WordWeb Online

1. Of great significance or value
   "important people"; "the important questions of the day"
   - of import
    
2. Important in effect or meaning
   - significant
    
3. Of extreme importance; vital to the resolution of a crisis
   - crucial
    
4. Having authority or ascendancy or influence
   "an important official"
   - authoritative
    
5. Having or suggesting a consciousness of high position
   "recited the decree with an important air"; "took long important strides in the direction of his office"

See also: all important, all-important, alpha, beta, big, burning, cardinal, central, chief, consequential, Copernican, critical, crucial, decisive, distinguished, earthshaking, earth-shattering, epochal, epoch-making, essential, eventful, evidential, evidentiary, fundamental, grand, grave, great, grievous, heavy, historic, immodest, importance, importantly, in-chief, influential, key, large, life-and-death, life-or-death, main, master, meaningful, measurable, momentous, monumental, most-valuable, noteworthy, of the essence, operative, outstanding, pivotal, polar, portentous, primal, primary, principal, probative, probatory, prodigious, profound, remarkable, serious, significance, strategic, valuable, weighty, world-shaking, world-shattering

Antonym: unimportant