To rejoin means to join again with something or someone you were separated from. When you leave your friends to get a snack and then come back, you rejoin the group. When a soccer player returns to the field after being injured, she rejoins her team.

The word captures that sense of reconnection: you were together, then apart, and now together again. A family might rejoin after everyone has gone their separate ways for work and school. Countries sometimes rejoin international organizations they had left. You might rejoin a conversation you stepped away from, picking up where things left off.

The word can also mean to reply or respond, especially with a quick or sharp answer. If someone criticizes your science project and you defend it firmly, you rejoin with your own point of view. This meaning is less common today but still appears in stories: “That's not fair!” she rejoined.

Notice how both meanings share the idea of returning or going back: you're either going back to a group physically, or going back at someone verbally with your response.