Meaning

「じゃあ」 is a casual form of それでは(soredewa).

We say 「じゃあ」 so many times in our conversation.

「それでは」 to me is associates with these English words or phrases:

“So....” “therefore” “ after all” , “then”, because of....”. “In which case..”

dialogue example:

A. いついける？ Itsu ikeru? when can you go?

B. 今日(きょう)はむり。 Kyou wa muri. Can’t today

A. じゃあ、あしたは？ Jaa, ashita wa? (In that case) how about tomorrow?

B. あしたもむり。Ashita mo muri. can’t go tomorrow either

A. じゃあ、いけるとき教(おし)えて。Jaa, ikeru toki oshiete. Then, just tell me when you can go.

Leave any comments if you have any question on this!