das Spiel
game
An activity with fixed rules that is done for fun, entertainment, or in competition.
Lass uns am Wochenende ein Brettspiel spielen.
Let's play a board game this weekend.

game / play
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game
An activity with fixed rules that is done for fun, entertainment, or in competition.
Lass uns am Wochenende ein Brettspiel spielen.
Let's play a board game this weekend.

'Das Spiel' (game/play) is the noun related to the very common verb 'spielen' (to play). 'Der Spieler' (the player) and 'das Spielzeug' (the toy) are also derived from it.
Die Kinder spielen am liebsten mit ihrem neuen Spielzeug.
The children like playing with their new toy most of all.
The idiom 'etwas aufs Spiel setzen' means to risk something important. The preposition 'aufs' is a fixed short form of 'auf das' and always requires the accusative.
Für diesen Traum hat er alles aufs Spiel gesetzt.
For this dream, he put everything at risk.
You can recognize the meaning of 'Spiel' from the context. Sports point to a competition, instruments to a musical performance, and words like 'Theater' (theater) to acting.
Das Spiel der deutschen Mannschaft war enttäuschend.
The German team's game was disappointing.
The most common combination is 'ein Spiel spielen' (to play a game). Even though it sounds as if you are repeating the same word, this is a completely normal and common expression.
Wollen wir heute Abend zusammen ein Spiel spielen?
Do we want to play a game together tonight?
When a sports event is meant, typical verbs are 'ansehen' (to watch), 'übertragen' (to broadcast), or 'pfeifen' (to referee / blow the whistle, said of the referee). The score is often described with 'das Spiel endet ...' (the game ends ...).
Der Schiedsrichter hat das Spiel pünktlich angepfiffen.
The referee kicked off the game right on time.
In a figurative sense, 'Spiel' often describes negative, manipulative actions. In this context, it is frequently combined with adjectives such as 'politisch' (political), 'gefährlich' (dangerous), or 'falsch' (false/dishonest).
Ich bin nicht bereit, dieses politische Spiel mitzuspielen.
I'm not willing to play along with this political game.
sentence / set
'das Spiel' and 'der Satz' can both occur in sports, but they do not mean the same thing. 'Spiel' means the whole game or match, or a single sporting contest. 'Satz' means only one section of play in sports such as tennis, table tennis, or volleyball. So you can win a match by winning several sets. The words are especially easy to confuse because both are mentioned when talking about the score. The simple difference is: the game or match is the whole thing; the set is one part of it.

Nach zwei Stunden gewann Lisa das Spiel gegen ihre starke Gegnerin.
After two hours, Lisa won the match against her strong opponent.
Im zweiten Satz spielte sie viel sicherer und machte kaum Fehler.
In the second set, she played much more steadily and made hardly any mistakes.