der Starter
participant
A person who takes part in a competition, especially a race.
Beim diesjährigen Marathon gab es über zehntausend Starter.
At this year's marathon, there were over ten thousand participants.

starter / appetizer
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participant
A person who takes part in a competition, especially a race.
Beim diesjährigen Marathon gab es über zehntausend Starter.
At this year's marathon, there were over ten thousand participants.

The word is derived from the verb 'starten' (to start). The ending '-er' indicates that it is a person or thing that performs an action — so someone or something that starts or starts something.
Der schnellste Starter gewann das Rennen.
The fastest starter won the race.
Because 'Starter' (starter / appetizer) has many meanings, context is crucial. Watch for signal words: 'Rennen' (race) or 'Wettkampf' (competition) (person), 'Menü' (menu) or 'bestellen' (to order) (food), 'Auto' (car) or 'Motor' (engine) (technical), and 'backen' (to bake) or 'Sauerteig' (sourdough) (bread).
Im Restaurant bestellte er einen Salat als Starter.
In the restaurant, he ordered a salad as an appetizer.
In this meaning, 'der Starter' (the participant) is a person who takes part in a competition. People often speak of the 'Anzahl der Starter' (number of participants) or a 'Feld von Startern' (field of participants).
Bei dem Rennen waren über fünfzig Starter gemeldet.
Over fifty participants were registered for the race.
The use of 'Starter' for 'Vorspeise' (appetizer) is modern and is often used in restaurants with international cuisine. It sounds a bit trendier than the classic German word.
Auf der Speisekarte stehen drei verschiedene Starter zur Auswahl.
There are three different appetizers to choose from on the menu.
In a sports context, you need to listen carefully: 'Der Starter' can mean the participant (meaning m01), but also the person who gives the starting signal. The latter is often made clear by the action: 'Der Starter hebt die Pistole' (the starter raises the pistol).
Alle Läufer blickten gespannt zum Starter.
All the runners looked expectantly toward the starter.
In a baking context, a sourdough starter is used with specific verbs. You can 'einen Starter ansetzen' (start a starter), 'füttern' (feed it with flour and water), or 'auffrischen' (refresh it).
Bevor du backen kannst, musst du den Starter aus dem Kühlschrank nehmen und füttern.
Before you can bake, you have to take the starter out of the fridge and feed it.
participant
'der Teilnehmer' and 'der Starter' can both refer to a person in a competition. 'Teilnehmer' is the general word for someone who takes part. 'Starter' fits especially races or competitions when someone is at the starting line or on the start list. But you have to be careful: 'der Starter' can also be the person who gives the starting signal. That is why 'Teilnehmer' is clearer and more general in many situations, while 'Starter' is more strongly connected to sports and the beginning of a competition.

Vor dem Marathon warten alle Starter ruhig hinter der Linie.
Before the marathon, all the starters wait calmly behind the line.
Heute bekommt jeder Teilnehmer des Wettbewerbs eine Nummer.
Today, each participant in the competition receives a number.