What speakers mean by it now
In ordinary English a month is one of the twelve named stretches of the civil year, or any similar span of about four weeks. Dictionaries also record looser uses: an indefinite stretch counted in months, and, in talk of pregnancy, one ninth of a typical term. None of that is exotic. The history underneath is. The origin of the word month is bound up with how people once counted time by watching the sky rather than a printed grid.
A native English noun
English did not borrow this word from French or Latin. It is inherited. Old English already had mōnaþ, also spelled with ð, a masculine noun for a month. Standard dictionaries place first English use before the twelfth century, which is simply the Old English period rather than a later loan. Middle English kept a cluster of spellings, among them moneth and month, before the shorter modern form won out. The long vowel of the first syllable shortened, the unstressed middle vowel dropped, and the final dental fricative stayed as th.
Old English did try a Latin rival. It borrowed calend from calendae, the Roman name for the first of a month and, by extension, a month itself. That loan did not last as the everyday unit. The native noun remained the word for the stretch of time; Latin later fed English calendar, kalends, and the bookkeeping of dates, not the ordinary name of the period.
Moon and month as a pair
Month and moon are related, not an accident of spelling. Old English mōna is the luminary; mōnaþ is the period. Both go back to Proto-Germanic. Comparative work reconstructs a stem on the order of *mēnan- or *menon- for the moon and *mēnōþs for the month. The West Germanic form *mānōþ is the immediate ancestor of the English noun.
That extra dental element is real morphology, not folk etymology. Germanic treated the counted interval as a derivative of the same root that named the disk in the sky. The two words stayed distinct. German still has Mond beside Monat; Dutch has maan beside maand; Danish has måne beside måned. Gothic already shows mēnōþs for the period. Icelandic mánuður belongs to the same set. English simply kept both members of the pair.
An older word that meant both things
Behind the Germanic forms sits Proto-Indo-European *mḗh₁n̥s, reconstructed with two senses at once: moon and month. Many specialists take it as a derivative of *meh₁-, a root meaning to measure, because lunations were an early and widespread way to mark time. That step is widely held and still marked as probable rather than proven. If the measure-link is right, the noun is not a label glued onto a planet after the fact; it is a measuring word that became a sky word, or both at once.
Outside Germanic, descendants often name only the calendar unit. Latin mensis is month and gives French mois, Italian mese, and Spanish mes. Ancient Greek mēn is month. Sanskrit māsa covers moon and month together. Old Irish mí and Welsh mis mean month. In Greek and Latin, new words took over for the moon itself: selēnē from a word for brightness, and luna. Surveys of the family agree on the pattern. In Greek, Italic, Celtic, and Armenian the old cognates typically mean month alone. Germanic kept related but separate stems for the light and the interval.
The same Indo-European material shows up still farther afield, in Baltic, Slavic, Indo-Iranian, and Tocharian forms for moon, month, or both. None of those are the source of the English word. They are siblings. English month is a Germanic inheritance from that shared ancestor, not a borrowing from Latin mensis, even though the two are cousins.
Lunation first, civil block later
The earliest English sense was not an arbitrary twelfth of a solar year. It was the wait from one new moon to the next. That interval, later called a synodic month, runs a little over twenty-nine and a half days. Late Old English already uses the noun that way. Twelve such lunations do not equal a solar year, so any community that planted by seasons had to add extra days, add an extra month, or watch named months drift against harvest.
The civil month is a later convenience: named blocks of twenty-eight to thirty-one days that add up to a year without tracking phases. English still needs extra labels when the two lengths matter. A lunar month follows the moon. A calendar month follows the almanac. Astronomers add still finer terms, such as sidereal month, for other circuits of the same body. The spelling of month did not change to mark the shift from lunation to civil block. Usage did.
Sense history has a few side paths. In early modern English the plural months could mean menstrual discharge, a use now obsolete in ordinary speech, though time of the month still glances at the same cycle. Colloquial English later added a month of Sundays for a very long time, a nineteenth-century exaggeration rather than a new technical sense.
Old English also built month-names as compounds on mōnaþ. Blōtmōnaþ named a late-autumn slaughter month; Hāligmōnaþ a holy month around what is now September; Sōlmōnaþ a muddy midwinter month. Those compounds show the noun as a working measure inside the year, not a borrowed heading on a Roman list.
The sky still sitting in the vocabulary
A useful last fact is the split that English did not make. Latin kept mensis for the period and used luna for the moon. Greek kept mēn for the period and used selēnē for the light. English kept moon and month, two native words from one Indo-European idea: time counted by a changing light. The printed calendar hid the phases. The pair of words did not.