To wane means to gradually decrease in size, strength, or intensity. The word is most famously used to describe the moon's monthly cycle: after the full moon, it begins to wane, appearing smaller each night until it disappears entirely into the new moon phase. The opposite is wax, when the moon grows larger night by night.

Waning happens in many contexts beyond the moon. A student might have waning enthusiasm for a project as the weeks drag on, starting strong but slowly fading. A fever wanes as you recover from illness, gradually losing its grip. Political leaders watch their popularity wax and wane depending on recent events. A successful sports team's fortunes might wane as their star players get older or retire.

The word carries a sense of something naturally running its course rather than stopping abruptly. A fad doesn't suddenly end. It wanes as fewer people care about it. When something wanes, it's like watching a candle burn lower and lower, the flame getting weaker but not yet gone.