Wane means to gradually decrease in intensity, strength, or power.
Wane is a verb.
Synonyms: ebb, fade, decrease, diminish, waste, wither, decline.
Antonyms: strengthen, advance, increase, rise, brighten, thrive, wax.
The moon appears to wax and wane according to the light it reflects.
Why is your enthusiasm for the event waning day by day?
My confidence in the project began to wane as we were running short of time.