hundertmal
a hundred times
Indicates that something happens or is repeated exactly one hundred times.
Der Rekordhalter konnte den Ball hundertmal auf dem Kopf jonglieren.
The record holder was able to juggle the ball on his head a hundred times.

a hundred times
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a hundred times
Indicates that something happens or is repeated exactly one hundred times.
Der Rekordhalter konnte den Ball hundertmal auf dem Kopf jonglieren.
The record holder was able to juggle the ball on his head a hundred times.

The word is a compound of the number word 'hundert' (hundred) and the ending '-mal', which indicates repetition. This pattern works with all numbers, e.g. 'zweimal' (twice), 'zehnmal' (ten times), 'tausendmal' (a thousand times).
Ich habe diesen Film bestimmt schon zehnmal gesehen.
I'm sure I've seen this film ten times already.
As an adverb that answers the question 'Wie oft?' (How often?), 'hundertmal' (a hundred times) usually stands in the middle of the sentence after the conjugated verb and the subject.
Er hat den Knopf hundertmal gedrückt, aber nichts ist passiert.
He pressed the button a hundred times, but nothing happened.
The literal meaning (exactly 100 times) is found in factual contexts such as records or counts. The exaggerated, colloquial meaning ('very often') can be recognized in emotional situations, often with reminders or complaints.
Der Athlet hat den Medizinball hundertmal über seinen Kopf gehoben.
The athlete lifted the medicine ball over his head a hundred times.
In spoken language, 'hundertmal' (a hundred times) is very often used as an exaggeration for 'very often' or 'again and again'. It often expresses impatience or frustration.
Warum fragst du mich das hundertmal? Ich habe schon geantwortet.
Why are you asking me that a hundred times? I've already answered.
often
'oft' and 'hundertmal' can both express that something happens many times. 'oft' is neutral and fits almost all situations when talking about frequency. In everyday speech, 'hundertmal' is usually an exaggeration: you do not mean exactly a hundred times, but very many times. It sounds stronger and often a bit more emotional, for example when someone is annoyed. For a calm, factual statement, 'oft' is more natural; for an emphasized statement, 'hundertmal' fits better.

Ich habe dir das schon hundertmal erklärt.
I've already explained that to you a hundred times.
Bei meinen Eltern essen wir oft zusammen zu Abend.
At my parents' place, we often have dinner together.