"On the wane" is a popular phrase used to describe a decline or decrease in something. However, there are several other synonyms that can be used in place of this phrase, such as "diminishing", "waning", "declining", "fading", "ebbing", or "waning away". Each of these phrases conveys the same meaning of something decreasing or fading away, but with a slightly different nuance. "Diminishing" suggests a steady reduction in strength or amount, while "fading" implies a gradual disappearance. "Ebbing" often describes something that comes and goes, like the tides. All of these words can be useful alternatives to "on the wane" in different contexts.