This was the depiction of East Asians as an existential threat to the Western World, mainly during the late 1800s and early 1900s.
Asians were used as antagonists in movies, novels, and comic books, often portrayed as criminal masterminds bent on taking over the world.
This was partially due to increased Asian immigration to the United States during this time, and the knowledge that the Asian population far outnumbered the population of the United States.
Why I Looked It Up
I was vaguely familiar with the term.
However, I read The Mystery of Dr. Fu-Manchu from 1913, and the phrase is explicitly used multiple times. The book is quite racist – in fact, the entire Fu Manchu series is cited as an example (often as the canonical example) in many discussions of Yellow Peril.
For example, after a long description of a terrifying criminal:
Imagine that awful being, and you have a mental picture of Dr. Fu-Manchu, the yellow peril incarnate in one man.
I was surprised by this, because I assumed that the word was pejorative, meaning a speaker wouldn’t admit to it and would take offense at being characterized by it. But, there it was – it was used quite matter of factly (six times, throughout the novel).