month

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A unit of time corresponding approximately to one cycle of the moon's phases, or about 30 days or 4 weeks.
• noun One of the 12 divisions of a year as determined by a calendar, especially the Gregorian calendar.
• noun A period extending from a date in one calendar month to the corresponding date in the following month.
• noun A sidereal month.
• noun A lunar month.
• noun A solar month.
• idiom (month of Sundays) An indefinitely long period of time.

from The Century Dictionary.

• noun Originally, the interval from one new moon to the next, called specifically a lunar, synodical, or illuminative month.
• noun One twelfth part of a tropical year, or 30 days, 10 hours, 29 minutes, 3. 8 seconds: called specifically a solar month.
• noun One of the twelve parts into which the calendar year is arbitrarily divided: called specifically a calendar month.
• noun At common law and in equity, month has been understood to mean ‘a lunar month,’ which is assumed to be 28 days, except when the contrary appears, and except when used of mercantile transactions, such as negotiable paper, etc.
• noun plural Same as menses. Minsheu; Cotgrave. Abbreviated mo.

from the GNU version of the Collaborative International Dictionary of English.

• noun One of the twelve portions into which the year is divided; the twelfth part of a year, corresponding nearly to the length of a synodic revolution of the moon, -- whence the name. In popular use, a period of four weeks is often called a month.
• noun [Obs.] A celebration made in remembrance of a deceased person a month after death.
• noun the months as adjusted in the common or Gregorian calendar; April, June, September, and November, containing 30 days, and the rest 31, except February, which, in common years, has 28, and in leap years 29.
• noun the period of one revolution of the moon, particularly a synodical revolution; but several kinds are distinguished, as the synodical month, or period from one new moon to the next, in mean length 29 d. 12 h. 44 m. 2.87 s.; the nodical month, or time of revolution from one node to the same again, in length 27 d. 5 h. 5 m. 36 s.; the sidereal, or time of revolution from a star to the same again, equal to 27 d. 7 h. 43 m. 11.5 s.; the anomalistic, or time of revolution from perigee to perigee again, in length 27 d. 13 h. 18 m. 37.4 s.; and the tropical, or time of passing from any point of the ecliptic to the same again, equal to 27 d. 7 h. 43 m. 4.7 s.
• noun the time in which the sun passes through one sign of the zodiac, in mean length 30 d. 10 h. 29 m. 4.1 s.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A period into which a year is divided, historically based on the phases of the moon. In the Gregorian calendar there are twelve months: January, February, March, April, May, June, July, August, September, October, November and December.
• noun A period of 30 days, 31 days, or some alternation thereof.
• noun obsolete, in the plural A woman's period; menstrual discharge.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a time unit of approximately 30 days
• noun one of the twelve divisions of the calendar year