Denshi Jisho — Online Japanese dictionary

一旦 いったん 1:
2: (Usually written using kana alone) once;
3: for a moment; temporarily; (Noun) (Archaism) one morning
Common word, Adverb 
一旦緩急 いったんかんきゅう (when) danger threatens; should an emergency occur
Expression 

You can also

Look up kanji details for 一旦.

Try a Yahoo Jisho search for 一旦 .
Try a Goo Jisho search for 一旦 .
Try a Google search for 一旦 .