die Besichtigung
viewing / tour
The careful looking at and inspecting of something, especially real estate, sights, or exhibitions.
Wir haben morgen eine Besichtigung für die neue Wohnung.
We have a viewing for the new apartment tomorrow.

viewing / tour
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viewing / tour
The careful looking at and inspecting of something, especially real estate, sights, or exhibitions.
Wir haben morgen eine Besichtigung für die neue Wohnung.
We have a viewing for the new apartment tomorrow.

'Besichtigung' is a noun derived from the verb 'besichtigen' (to view/visit). The ending '-ung' is a typical way to form feminine nouns from verbs that describe an action or its result.
Die Besichtigung des Schlosses dauert etwa eine Stunde.
The tour of the castle takes about an hour.
Common verbs used in connection with 'Besichtigung' are 'eine Besichtigung vereinbaren' (to arrange a viewing), 'eine Besichtigung machen' (to do/go on a viewing), or 'eine Besichtigung durchführen' (to conduct a viewing/tour).
Haben Sie schon einen Termin für die Wohnungsbesichtigung vereinbart?
Have you already arranged an appointment for the apartment viewing?
A very common and useful compound word is 'der Besichtigungstermin' (viewing appointment). It refers to the fixed time when you go to look at something, especially an apartment or house.
Unser Besichtigungstermin ist am Freitag um 15 Uhr.
Our viewing appointment is on Friday at 3 p.m.
visit / visitor
'der Besuch' and 'die Besichtigung' can both mean that you go to a place. 'der Besuch' (visit) is more general: you go to a person, a place, or an event and stay there for a certain amount of time. 'die Besichtigung' (viewing / tour) is more specific: you deliberately look at something closely, for example an apartment, a museum, or a tourist sight. Learners may confuse the words because both have to do with going somewhere. If looking at and inspecting something is the focus, 'Besichtigung' fits better. If it is just about going there or spending time together, 'Besuch' is more natural.

Vor dem Umzug haben wir eine Besichtigung der neuen Wohnung vereinbart.
Before moving, we arranged a viewing of the new apartment.
Der Besuch bei meiner Tante hat länger gedauert als geplant.
The visit to my aunt took longer than planned.