1. A unit of time corresponding approximately to one cycle of the moon's phases, or about 30 days or 4 weeks.
2. (Abbr. mo.) 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.

1. An indefinitely long period of time: It will take you a month of Sundays to chop all that wood.

[Middle English moneth, from Old English mōnath.]

USAGE NOTE   The singular month, preceded by a number and a hyphen, is used as a compound attributive: a three-month vacation. The plural possessive form without a hyphen is also possible: a three months' vacation.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.