abgeschlossen
completed
Ended, finished, or completed.
Nach fünf Jahren hat sie ihr Medizinstudium erfolgreich abgeschlossen.
After five years, she successfully completed her medical studies.

completed / locked
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completed
Ended, finished, or completed.
Nach fünf Jahren hat sie ihr Medizinstudium erfolgreich abgeschlossen.
After five years, she successfully completed her medical studies.

'Abgeschlossen' (completed/locked) is the past participle of the separable verb 'abschließen' (to complete/to lock). The meanings of the adjective are derived from the meanings of the verb: to finish something, to lock something, or to separate something off.
Ein abgeschlossenes Kapitel meines Lebens liegt nun hinter mir.
A completed chapter of my life is now behind me.
In the meaning 'beendet' (finished), 'abgeschlossen' (completed) is often used for processes with a clear end, such as 'ein abgeschlossenes Studium' (completed university studies), 'eine abgeschlossene Ausbildung' (completed vocational training), or 'ein abgeschlossenes Projekt' (a completed project).
Für diesen Job benötigen Sie eine abgeschlossene Berufsausbildung.
For this job, you need completed vocational training.
This meaning always refers to physical objects that can be locked. One speaks of an 'abgeschlossene Tür' (locked door), an 'abgeschlossenes Fenster' (locked window), or an 'abgeschlossenes Auto' (locked car).
Bewahren Sie Ihre Wertsachen in einem abgeschlossenen Schrank auf.
Keep your valuables in a locked cabinet.
In the context of real estate, 'abgeschlossen' (self-contained) almost always means 'independent' or 'separate'. An 'abgeschlossene Wohnung' (self-contained apartment) has its own entrance, its own kitchen, and its own bathroom.
Die abgeschlossene Wohnung im Dachgeschoss ist ideal für eine Person.
The self-contained apartment in the attic is ideal for one person.
This meaning describes a person or place that is isolated from the outside world. A typical phrase is 'abgeschlossen leben' (to live secluded).
Seit dem Tod seiner Frau lebt er sehr abgeschlossen und meidet den Kontakt zu anderen.
Since his wife's death, he has lived very secluded and avoids contact with others.
round / about
'rund' and 'abgeschlossen' can both have to do with something being finished. 'abgeschlossen' mainly means that a piece of work, a project, or a process has been completed. 'rund' says more about the impression something gives: it is not only finished, but feels coherent and well put together. A project can be 'abgeschlossen' even if the result is not quite 'rund'. So 'abgeschlossen' emphasizes the ending, while 'rund' emphasizes the successful overall form.

Im Büro ist das Projekt seit gestern abgeschlossen.
At the office, the project has been completed since yesterday.
Mit den neuen Bildern sieht die Präsentation viel runder aus.
With the new images, the presentation looks much more polished.