Onset and Starting point

Onset noun - The act or action of setting upon with force or violence.

Starting point and onset are semantically related in beginning topic. In some cases you can use "Starting point" instead a noun "Onset".

Starting point noun - Earliest limiting point.

Onset and starting point are semantically related in beginning topic. You can use "Onset" instead a noun phrase "Starting point", if it concerns topics such as start of an event or action, ceremony marking the beginning of stage.