der Spieler
player
A person who takes part in a game or sport.
Der verletzte Spieler musste ausgewechselt werden.
The injured player had to be substituted.

player
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player
A person who takes part in a game or sport.
Der verletzte Spieler musste ausgewechselt werden.
The injured player had to be substituted.

'Der Spieler' (the player) is derived directly from the verb 'spielen' (to play). The ending '-er' refers to a person who performs the action of the verb. This structure is very common in German.
Die Kinder spielen im Garten.
The children are playing in the garden.
In English, 'player' can colloquially mean a womanizer. The German word 'Spieler' does not have this negative meaning.
Er ist ein sehr guter Spieler in unserer Fußballmannschaft.
He is a very good player on our soccer team.
The word 'Spieler' (player) is often part of compound nouns that describe the person's role more precisely. Examples are 'Mitspieler' (teammate/fellow player), 'Gegenspieler' (opponent), or 'Nationalspieler' (national team player).
Sein Gegenspieler war viel schneller als er.
His opponent was much faster than he was.
This meaning becomes clear through the context of gambling, betting, or casinos. Without such a clue, 'Spieler' (player) is usually understood as someone taking part in a sport or game.
Er hat beim Pokern viel Geld verloren, er ist ein echter Spieler.
He lost a lot of money playing poker; he is a real gambler.
This meaning is metaphorical and is usually used in the context of business or politics. The person or company is often described as a 'großer Spieler' (big player) or 'wichtiger Spieler' (important player).
In der Technologiebranche gibt es nur wenige große Spieler.
In the technology sector, there are only a few big players.
When a technical device is meant, 'Spieler' (player) almost never stands alone. It is always part of a compound word such as 'CD-Spieler' (CD player), 'Plattenspieler' (record player), or 'MP3-Spieler' (MP3 player).
Ich höre gerne Musik auf meinem alten Plattenspieler.
I like listening to music on my old record player.
This meaning of 'Schauspieler' (actor) is outdated or belongs to very elevated, literary language. It is not used in normal everyday speech.
Die Spieler auf der Bühne verbeugten sich vor dem Publikum.
The actors on the stage bowed to the audience.
participant
'der Spieler' and 'der Teilnehmer' both refer to people who take part in something. 'Spieler' is used when it is really about a game or a sport, for example soccer, tennis, or a board game. 'Teilnehmer' is more general and fits courses, events, competitions, or meetings. The words can be confused because a player is also a participant in a game. But in a language course or a meeting, you would normally say 'Teilnehmer', not 'Spieler'.

Nach dem Foul spricht der Schiedsrichter mit dem Spieler.
After the foul, the referee speaks with the player.
Am Wochenende begrüßt Lukas die neuen Teilnehmer im Kochkurs.
On the weekend, Lukas welcomes the new participants in the cooking course.