Vollendung
completion
The act of finishing something or the successful conclusion of a process, plan, or work.
Die Vollendung des Bauprojekts wird noch mehrere Monate dauern.
The completion of the construction project will still take several months.

completion / perfection
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completion
The act of finishing something or the successful conclusion of a process, plan, or work.
Die Vollendung des Bauprojekts wird noch mehrere Monate dauern.
The completion of the construction project will still take several months.

'Vollendung' (completion/perfection) is derived from the verb 'vollenden' (to complete), which consists of 'voll' (full) and 'enden' (to end). It literally describes the act of bringing something to a 'full end,' that is, completing it fully.
Der Künstler arbeitete jahrelang an der Vollendung seines Meisterwerks.
The artist worked for years on the completion of his masterpiece.
Common phrases connected with the completion of a process are 'etwas zur Vollendung bringen' (to bring something to completion) or 'vor der Vollendung stehen' (to be nearing completion).
Der Architekt will das Gebäude noch in diesem Jahr zur Vollendung bringen.
The architect wants to bring the building to completion this year.
To express the highest perfection, 'Vollendung' (perfection) is often used with the preposition 'in' or 'mit' (with). The phrase 'in Vollendung' then means something like 'perfectly' or 'to perfection.'
Er beherrscht die Kunst des Geigenspiels in Vollendung.
He has mastered the art of violin playing to perfection.
beginning
'der Anfang' and 'die Vollendung' are clear opposites. 'der Anfang' is the first part or the beginning of something. 'die Vollendung' is the moment or state in which something has been completed. So the beginning comes before the process, while completion comes after the successful conclusion. 'Anfang' fits when something is just beginning. 'Vollendung' is more suitable for a work, plan, or longer process that is being completed.

Die Vollendung des Hauses dauert noch einige Wochen.
The completion of the house will still take a few weeks.
Tim liest im Bus den Anfang des Berichts.
Tim reads the beginning of the report on the bus.