Onset and Proem

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

Proem and onset are semantically related. In some cases you can use "Proem" instead a noun "Onset".

Proem noun - A short section (as of a book) that leads to or explains the main part.

Onset and proem are semantically related. Sometimes you can use "Onset" instead a noun "Proem".