The '-ung' ending is a common way of making a verb into a noun. The verb is 'entschuldigen' (to excuse) and you're right - this would work as well.
To say "Excuse me!", you can say either "Entchuldigung!" or "Entschuldigen Sie!".
Why is it (Entschuldigung) considered a noun (capitalized), wouldn't it work as a verb?
Why can't we use the singular form?