on Writers Helping Writers:

Purple prose is one of those writing problems that’s easy to see in someone else’s work but hard to spot in our own. Simply put, this writing is overly elaborate or self-indulgent. It’s wordy, heavy on description, and often prioritizes the sound of the words over clarity and pacing. The result? Writing so ornate, flowery, or dramatic that it calls attention to itself and gets in the way of the story.

This kind of writing can be found in long passages (a full page describing a sunset) or in small samples (an overwrought metaphor or sentence in an otherwise well-written scene). Either way, it’s a sign the writing (and the author) is trying hard to impress.