das Fenster
window
An opening in a wall, usually fitted with glass, that lets light and air into a room.
Mach bitte das Fenster auf, es ist sehr warm hier drin.
Please open the window; it's very warm in here.

window
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window
An opening in a wall, usually fitted with glass, that lets light and air into a room.
Mach bitte das Fenster auf, es ist sehr warm hier drin.
Please open the window; it's very warm in here.

Pay attention to the prepositions: 'ans Fenster' (to the window, with accusative) describes movement toward the window, while 'am Fenster' (at/by the window, with dative) describes a position.
Er geht ans Fenster, um die Vögel zu beobachten.
He goes to the window to watch the birds.
In connection with a real window, the verbs 'öffnen' (to open), 'schließen' (to close), and 'putzen' (to clean) are often used. Colloquially, you also hear 'aufmachen' (to open) and 'zumachen' (to close).
Könntest du bitte das Fenster schließen? Es zieht.
Could you please close the window? There's a draft.
When talking about computers, programs, or the internet, 'Fenster' means a separate area on the screen. Typical verbs here are 'öffnen' (to open), 'schließen' (to close), or 'minimieren' (to minimize).
Schließe bitte alle unnötigen Fenster, damit der Computer schneller wird.
Please close all unnecessary windows so the computer gets faster.
In the figurative meaning, 'Fenster' describes a limited period of time. It is often used as 'Zeitfenster' (time window) to make this clear.
Wir haben nur ein kleines Zeitfenster, um das Projekt erfolgreich abzuschließen.
We have only a small time window to complete the project successfully.
appointment / deadline
'das Fenster' and 'der Termin' can both have to do with time, but they do not mean the same thing. In this meaning, 'Fenster' refers to a limited period of time in which something is possible or favorable; people often say 'Zeitfenster' (time window). A Termin is a fixed point in time for a meeting or event, and sometimes also a deadline. The words can be confused because both are used when planning. A Zeitfenster is more of a window of opportunity or flexible time slot; a Termin is more of a specific time.

Am Montag haben wir nur ein kleines Zeitfenster für die Lieferung.
On Monday we only have a small time window for the delivery.
Mein Termin beim Arzt ist morgen um zehn Uhr.
My doctor's appointment is tomorrow at ten o'clock.