momentan
currently
At the present time; currently or at the moment.
Momentan habe ich leider keine Zeit für ein Treffen.
Unfortunately, I currently don't have time for a meeting.

currently
Loading your saved state.
currently
At the present time; currently or at the moment.
Momentan habe ich leider keine Zeit für ein Treffen.
Unfortunately, I currently don't have time for a meeting.

The adverb 'momentan' (currently) is derived directly from the noun 'der Moment' (the moment). So it describes a state that is valid for the present moment.
Einen Moment bitte, ich bin gleich für Sie da.
One moment please, I'll be right with you.
'Momentan' means 'zurzeit' (at the moment) or 'gegenwärtig' (currently). Don't confuse it with the English 'momentarily', which often means 'in Kürze' (in a moment).
Die Chefin ist momentan in einer Besprechung.
The boss is currently in a meeting.
As an adverb, 'momentan' (currently) is flexible. It can stand at the beginning of the sentence in position 1 (then the verb follows), or in the middle field of the sentence, usually after the verb and the subject.
Momentan ist das Restaurant geschlossen.
The restaurant is currently closed.
You can recognize the physical-technical meaning from the context. It appears almost exclusively in technical language, often in connection with measured values such as 'Geschwindigkeit' (speed), 'Leistung' (power), or 'Beschleunigung' (acceleration).
Der Sensor misst den momentanen Druck im System.
The sensor measures the instantaneous pressure in the system.
now
'jetzt' and 'momentan' are close in meaning because both point to the present. 'jetzt' can refer to the exact moment and sounds very direct. 'momentan' is more about a current situation that applies at the moment but may change later. In everyday language, you can often use both. 'Momentan' sounds especially natural when talking about availability, plans, work, or a temporary situation.

Der Gast kann momentan nicht bezahlen, weil seine Karte nicht funktioniert.
The guest currently can't pay because his card isn't working.
Ich buche jetzt die Fahrkarten für die Reise.
I'm booking the tickets for the trip now.