means One of the twelve divisions of a year, roughly thirty days long, used to organize time and approximately tied to the cycles of the moon.

from From Old English 'mōnaþ,' which is the same word as 'mōna,' the moon — because a month was originally one lunar cycle, the time from one new moon to the next. It traces back to Proto-Germanic 'mēnōth' and ultimately a Proto-Indo-European root tied to both 'moon' and 'measure,' since the moon was humanity's first calendar. The lunar month runs about 29.5 days, which is precisely why our tidied-up calendar months no longer match the sky.