A period into which a year is divided, historically based on the phases of the moon.
"The ancient calendar originally structured every month around the waxing and waning cycles of the moon rather than fixed dates."
In plain English: A month is a period of time that lasts about four weeks and is used to count the days on a calendar.
"We will finish this project by the end of the month."
Usage: Use "month" to refer to any of the twelve roughly 30- or 31-day periods that make up a calendar year. Do not use it to describe a specific date within that period, such as saying "on month five."
The word "month" comes from Old English and originally meant "moon," reflecting the ancient practice of measuring time by lunar cycles. It traveled into modern English through Middle English while retaining its connection to the phases of the moon.