じゃあ

also: じゃ

じゃ

あ

Meanings

1

Casual conjunction used to start a new topic, conclude a conversation, or indicate a logical consequence. Derived from では.

じゃあ、またsee you then; used to say goodbye implying a future meeting
じゃあ、また来らい週しゅう。
Okay then, see you next week.
じゃあねcasual way to say 'bye' at the end of a conversation
じゃあね、気きをつけて。
Bye then, take care.
じゃあ、〈動どう詞し〉しようthen let's ...; used to suggest an action based on what was just said
じゃあ、そろそろ出しゅっ発ぱつしよう。
Well then, let's get going soon.

それならN4
それなら explicitly means 'if that's the case', while じゃあ is a looser, more casual filler that can also mark a new topic or closing remark.
そしたら
そしたら is an equally casual way to say 'so then', often used interchangeably with じゃあ in spoken Japanese but leaning more toward a sequential result.
んじゃ
じゃあ is a common casual way to say bye, while んじゃ is an even more contracted, colloquial variant.
それじゃN3
じゃあ is an even more casual, shortened form than それじゃ, often standing alone to mean 'well then' or 'bye'.
それではN4
じゃあ is an even more casual, everyday version of それでは, often used to signal a decision or to say goodbye informally.

ではpolite 丁寧語N5
では is the more formal, written counterpart of じゃあ, which is the casual spoken form used in everyday conversation.

Colloquial contraction of では.