der Zuschauer
spectator / viewer
A person who watches an event, performance, or broadcast.
Die Zuschauer im Stadion jubelten ihrer Mannschaft zu.
The spectators in the stadium cheered for their team.

spectator / viewer
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spectator / viewer
A person who watches an event, performance, or broadcast.
Die Zuschauer im Stadion jubelten ihrer Mannschaft zu.
The spectators in the stadium cheered for their team.

'Zuschauer' (spectator/viewer) is derived from the separable verb 'zuschauen' (to watch). The prefix 'zu-' indicates turning toward something, so 'to look in a direction.' A Zuschauer is therefore a person who deliberately watches something.
Ich schaue gerne den Wolken am Himmel zu.
I like watching the clouds in the sky.
The word is used for people at many different kinds of events. Typical places are 'im Stadion' (in the stadium), 'im Theater' (in the theater), 'im Kino' (in the cinema), or also 'vor dem Fernseher' (in front of the TV).
Die Sendung hatte gestern Abend Millionen von Zuschauern.
The show had millions of viewers last night.
Spectators are not always passive. Common verbs that describe what spectators do are 'jubeln' (to cheer), 'applaudieren' (to applaud), or 'mitfiebern' (to be emotionally involved / to root along).
Nach dem Tor jubelten die Zuschauer laut.
After the goal, the spectators cheered loudly.
participant
'der Teilnehmer' and 'der Zuschauer' are both connected to events, courses, games, or competitions. The important difference is the role: a participant actively takes part, while a spectator only watches. At a sports festival, for example, the participant runs in the race, while the spectator stands at the side. The words can be confused because both people can be in the same place. Memory tip: participants act; spectators observe.

Viele Zuschauer standen am Rand und klatschten für die Läufer.
Many spectators stood at the side and clapped for the runners.
Jeder Teilnehmer bekommt vor dem Lauf eine Startnummer.
Every participant gets a race number before the run.