What literary device is A common thread or repeated idea in a piece of writing?

The literary device you're describing is called a motif.

Here's a breakdown:

* Motif: A recurring image, symbol, theme, idea, or character type that appears throughout a work of literature, often serving to emphasize a central idea or theme.

Think of it like a repeated melody in a song - it's not the whole song, but it's a key element that adds to the overall meaning and impact.

Let me know if you'd like examples of motifs in literature!