der Antritt
start
The beginning or taking up of an activity, office, or journey.
Der Antritt seiner neuen Stelle als Direktor wurde mit einer kleinen Feier begangen.
The start of his new position as director was marked with a small celebration.

start / acceleration
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start
The beginning or taking up of an activity, office, or journey.
Der Antritt seiner neuen Stelle als Direktor wurde mit einer kleinen Feier begangen.
The start of his new position as director was marked with a small celebration.

'Antritt' (start / acceleration) is the noun form of the verb 'antreten' (to take up / to begin / to step up). The verb literally means 'to step up to something' or 'to begin something'. This idea of the 'first step' explains both the beginning of an office and acceleration in sports.
Der neue Präsident wird morgen sein Amt antreten.
The new president will take office tomorrow.
The context makes the meaning clear. If it is about an office, a position, or a journey, it means the beginning. If it is about sports and speed, it means quick acceleration.
Vor dem Antritt seiner Weltreise musste er noch viel organisieren.
Before the start of his trip around the world, he still had a lot to organize.
In the meaning 'Beginn' (beginning), 'Antritt' (start) is very often combined with a noun in the genitive that describes the activity or position more precisely. Typical examples are 'der Antritt des Dienstes' (the start of duty) or 'der Antritt der Nachfolge' (the assumption of succession).
Der Antritt seines neuen Amtes wurde in der Presse ausführlich kommentiert.
The start of his new office was covered in detail in the press.
In a sports context, 'Antritt' (acceleration) is often combined with adjectives that describe the quality of the acceleration. Common combinations are 'ein schneller Antritt' (quick acceleration), 'ein explosiver Antritt' (explosive acceleration), or 'ein starker Antritt' (strong acceleration).
Dank seines starken Antritts konnte der Sprinter das Rennen gewinnen.
Thanks to his strong acceleration, the sprinter was able to win the race.
beginning
'der Beginn' and 'der Antritt' can both mean the start of something. 'der Beginn' is general and fits almost all events, actions, and periods of time. 'der Antritt' is more specific: it is used especially when someone takes up a position, an office, or a journey. That is why people say, for example, 'der Antritt einer neuen Stelle' (the start of a new position) or 'der Antritt einer Reise' (the start of a journey). The words can be confused because both describe a beginning. As a practical rule: for normal points in time, you usually use 'Beginn'; for taking up a task, an office, or a journey, 'Antritt' is more likely.

Beim Antritt seiner neuen Stelle war Max sehr nervös.
When Max started his new position, he was very nervous.
Vor Beginn der Prüfung müssen alle Handys ausgeschaltet sein.
Before the beginning of the exam, all cell phones must be switched off.