Wordnet 3.0
ADVERB (1)
1. 
 used in negative statement to describe a situation that has existed up to this point or up to the present time; 
- Example: "So far he hasn't called"- Example: "the sun isn't up yet"[syn: so far, thus far, up to now, hitherto, heretofore, as yet, yet, til now, until now]
WordNet (r) 3.0 (2006):
until now
    adv 1: used in negative statement to describe a situation that
           has existed up to this point or up to the present time;
           "So far he hasn't called"; "the sun isn't up yet" [syn:
           so far, thus far, up to now, hitherto,
           heretofore, as yet, yet, til now, until now]