der Auslöser
trigger
The immediate cause or reason that sets an event, action, or reaction in motion.
Der Auslöser für den Streit war eine unbedachte Bemerkung.
The trigger for the argument was a careless remark.

trigger
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trigger
The immediate cause or reason that sets an event, action, or reaction in motion.
Der Auslöser für den Streit war eine unbedachte Bemerkung.
The trigger for the argument was a careless remark.

'Der Auslöser' (the trigger) is the noun derived from the verb 'auslösen' (to trigger/to cause). The ending '-er' here refers to the thing or mechanism that triggers something.
Ein lauter Knall löste den Alarm aus.
A loud bang set off the alarm.
The context helps you recognize the meaning. In connection with cameras and photography, it means the button. With conflicts or events, it means the cause. With technical devices such as traps or alarms, it is a mechanism.
Der Auslöser der Kamera klemmt.
The camera's shutter button is stuck.
In the meaning 'Ursache' (cause), 'Auslöser' (trigger) is very often used with the preposition 'für' (for) and the accusative. The pattern is: der Auslöser für etwas sein (to be the trigger for something).
Stress kann ein Auslöser für viele Krankheiten sein.
Stress can be a trigger for many illnesses.
In the context of photography, the fixed combination 'den Auslöser drücken' (to press the shutter button) is very common. It means to take a photo.
Ich habe im richtigen Moment den Auslöser gedrückt.
I pressed the shutter button at the right moment.
reason / ground
'der Grund' and 'der Auslöser' can both name a cause. 'der Grund' is more general and can be an explanation, an occasion, or a justification. 'der Auslöser' is more specific: it is the direct moment or the direct thing that sets something in motion. In an argument, for example, the 'Grund' might be an old problem, but the 'Auslöser' might be a single sentence. Learners confuse the words because both can answer the question 'Warum?' (why?). If you mean the general cause, use 'Grund'. If you mean the immediate starting point of a reaction, 'Auslöser' fits better.

Ein lautes Geräusch im Park war der Auslöser für die Panik.
A loud noise in the park was the trigger for the panic.
Der Grund für die Absage ist noch nicht bekannt.
The reason for the cancellation is not yet known.