What does "onlooker" mean?

A spectator; someone looks on or watches, without becoming involved or participating.

What is a synonym for "onlooker"?

Common synonyms include: abutter, adjoiner, analyst, beholder, beholders, bird-watcher.

Pronounced //ˈɒnˌlʊk.ə(ɹ)//.

/ˈɒnˌlʊk.ə(ɹ)/ noun

noun ·Uncommon ·Advanced level

Noun

1
A spectator; someone looks on or watches, without becoming involved or participating.
"I wasn’t involved in the fight; I was only an onlooker."
2
someone who looks on wordnet

"This onlooker almost got in the way of the fire truck."

From on + look + -er, probably from the verb look on, but compare with onlook.