When there's steadily less and less of something, it's dwindling. If you have a dwindling supply of candy on Halloween, you may need to run to the store for more fun-size candy bars!
As a noun, dwindling means a state of becoming less, like the dwindling of your bank account after you quit your job. Use it as an adjective whenever something is growing smaller or sparser, like the dwindling leaves on the oak tree in autumn or the dwindling hair on your dad's head. Dwindling is from dwindle, which Shakespeare coined in the 1590s from dwine, "waste or pine away."