それじゃ

それじゃ
conjunction, interjection
well then; in that case
informalcolloquialexpressiongreeting
See also: それでは (formal form), じゃあ (more casual form)
1. well then; in that case; so
casual contraction of それでは; used to transition to a new topic, make a conclusion, or indicate departure
それじゃ、また明日(あした)
Well then, see you tomorrow.
それじゃ、(はじ)めましょう
Alright, let's begin.
それじゃダメだ
That won't do / That's no good.

Casual contraction

それじゃ is a casual contraction of それ:

Usages

1. DEPARTURE: それじゃ、(かえ)ります (Well, I'm heading home)
2. TRANSITION: それじゃ、(つぎ)話題(わだい) (So, on to the next topic)
3. CONSEQUENCE: それじゃ(こま) (That would be a problem)
4. STARTING: それじゃ、(はじ)めよう (Alright, let's start)

Formality scale