A lunar phenomenon that takes place during autumn, roughly at harvest time. The moon is much closer to the earth at that point, and takes on a brownish-yellow hue, presumably due to the dust in the earth's stratosphere.
Harvest Moon is also the US name of a series of farming simulations made in Japan. However, this does not apply to the games in their native country, where the series is called Bokujou Monogatari (or The Farm Story).
The harvest moon shines brightest as the crops begin to ripen.