Your cousin's daughter is your daughter's second cousin.
Here's how the relationship works:
First cousins share a set of grandparents. So, you and your cousin share the same set of grandparents.
Your cousin's daughter is the child of your cousin, which makes her your second cousin. You and your cousin's daughter have great-grandparents in common.
To summarize:
- You and your cousin have grandparents in common (your parents' parents).
- Your cousin's daughter and you have great-grandparents in common (your grandparents' parents).
Therefore, your cousin's daughter is your second cousin.