Answer by Professional Japanese Teacher
Tadaima means “I’m home” in Japanese. It is actually a shortened version of the phrase “ただいま 帰(かえ)りました tadaima kaerimashita”, meaning “I have just come back home now”.
The literal translation of tadaima is “just now”. It is usually said when you have come back home.
If you’re at home and your family member says “Tadaima” upon coming back, you can say “お帰(かえ)り okaeri” or “お帰(かえ)りなさい kaerinasai”. It means “welcome back”.