1. A unit of time corresponding approximately to one cycle of the moon's phases, or about 30 days or 4 weeks.
2. One of the 12 divisions of a year as determined by a calendar, especially the Gregorian calendar. Also called calendar month.
3. A period extending from a date in one calendar month to the corresponding date in the following month.
4. A sidereal month.
5. A lunar month.
6. A solar month.
month of Sundays Informal
An indefinitely long period of time: It will take you a month of Sundays to chop all that wood.
[Middle English moneth, month, from Old English mōnath; see mē-2 in the Appendix of Indo-European roots.]
Usage Note: The singular noun month, preceded by a number and a hyphen, is used as a compound adjective: a three-month vacation. The plural possessive form without a hyphen is also standard: a three months' vacation.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.