An interjection/adverb meaning 'now then', 'when the time comes', or 'come on'—used to urge action or mark the moment to begin or decide.
いざ、出発だ！忘れ物はないか？
Now then, let's depart! Did you forget anything?
いざとなったら、助けを呼んでください。
If it comes to that, please call for help.
いざ勝負！自分を信じてやってこい。
Now it's time to fight! Believe in yourself and go for it.