
abnehmend
decreasing
Becoming smaller in amount, size, strength, or intensity; becoming less.
Das Interesse an der Veranstaltung war in den letzten Wochen stark abnehmend.
Interest in the event had been sharply decreasing in recent weeks.
decreasing / waning
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decreasing
Becoming smaller in amount, size, strength, or intensity; becoming less.
Das Interesse an der Veranstaltung war in den letzten Wochen stark abnehmend.
Interest in the event had been sharply decreasing in recent weeks.

waning
Describes the phase of the moon after the full moon, during which the visible, illuminated part becomes smaller.
Bei abnehmendem Mond soll man am besten die Fenster putzen.
People say it is best to clean the windows during a waning moon.
'Abnehmend' (decreasing / waning) is the present participle of the separable verb 'abnehmen' (to decrease / to lose weight). The verb means 'to become less' or 'to lose weight', and this meaning transfers directly to the adjective.
Die Zahl der Teilnehmer nimmt seit Wochen ab.
The number of participants has been decreasing for weeks.
You can use 'abnehmend' (decreasing / waning) like a normal adjective before a noun, or use it as an adverb to describe an event or development. In the first case it is declined; in the second it remains unchanged.
Eine abnehmende Nachfrage führt zu sinkenden Preisen.
Decreasing demand leads to falling prices.
In its general meaning, 'abnehmend' (decreasing) is often combined with abstract concepts. Typical phrases are 'abnehmendes Interesse' (decreasing interest), 'abnehmende Kraft' (decreasing strength), or 'abnehmende Bedeutung' (decreasing importance).
Mit abnehmender Kraft konnte der Sportler das Tempo nicht mehr halten.
With decreasing strength, the athlete could no longer maintain the pace.
You can almost always recognize this special meaning from the word 'Mond' (moon). The phrase 'abnehmender Mond' (waning moon) is a fixed collocation in astronomy and folk belief.
Der Kalender zeigt an, wann zunehmender und abnehmender Mond ist.
The calendar shows when the moon is waxing and waning.
declining / retrograde
Rückläufig and abnehmend both mean that something is becoming less. Rückläufig is preferred for a statistical or longer-term trend and sounds more formal. Abnehmend focuses more generally on the process of reduction itself.

Im Sommer bemerkte der Händler abnehmende Verkaufszahlen.
In the summer, the retailer noticed that sales figures were decreasing.
Der Jahresbericht zeigt seit fünf Jahren rückläufige Verkaufszahlen.
The annual report shows declining sales figures over the past five years.