Anomalistic month

see under (def 5).
also called calendar month. any of the twelve parts, as january or february, into which the calendar year is divided.
the time from any day of one to the corresponding day of the next.
a period of four weeks or 30 days.
also called solar month. one-twelfth of a solar or tropical year.
also called lunar month. the period of a complete revolution of the moon around the earth, as the period between successive new moons (synodic month) equal to 29.531 days, or the period between successive conjunctions with a star (sidereal month) equal to 27.322 days, or the period between successive perigees (anomalistic month) equal to 27.555 days, or the period between successive similar nodes (nodical month or draconic month) equal to 27.212 days.
an unusually long period of time of indefinite length:
i haven’t seen him for months.
a month of sundays. (def 4)
noun
the interval between two successive p-ssages of the moon through perigee; 27.55455 days
noun
one of the twelve divisions (calendar months) of the calendar year
a period of time extending from one date to a corresponding date in the next calendar month
a period of four weeks or of 30 days
the period of time (tropical month) taken by the moon to return to the same longitude after one complete revolution around the earth; 27.321 58 days (approximately 27 days, 7 hours, 43 minutes, 4.5 seconds)
the period of time (sidereal month) taken by the moon to make one complete revolution around the earth, measured between two successive conjunctions with a distant star; 27.321 66 days (approximately 27 days, 7 hours, 43 minutes, 11 seconds)
also called lunation. the period of time (lunar or synodic month) taken by the moon to make one complete revolution around the earth, measured between two successive new moons; 29.530 59 days (approximately 29 days, 12 hours, 44 minutes, 3 seconds)
(informal) a month of sundays, a long unspecified period
n.

old english monað, from proto-germanic -menoth- (cf. old saxon manoth, old frisian monath, middle dutch manet, dutch maand, old high german manod, german monat, old norse manaðr, gothic menoþs “month”), related to -menon- “moon” (see moon (n.); the month was calculated from lunar phases). its cognates mean only “month” in the romance languages, but in germanic generally continue to do double duty. phrase a month of sundays “a very long time” is from 1832 (roughly 7 and a half months, but never used literally).

among the egyptians the month of thirty days each was in use long before the time of the exodus, and formed the basis of their calculations. from the time of the inst-tution of the mosaic law the month among the jews was lunar. the cycle of religious feasts depended on the moon. the commencement of a month was determined by the observation of the new moon. the number of months in the year was usually twelve (1 kings 4:7; 1 chr. 27:1-15); but every third year an additional month (ve-adar) was inserted, so as to make the months coincide with the seasons. “the hebrews and phoenicians had no word for month save ‘moon,’ and only saved their calendar from becoming vague like that of the moslems by the interpolation of an additional month. there is no evidence at all that they ever used a true solar year such as the egyptians possessed. the latter had twelve months of thirty days and five epagomenac or odd days.”, palestine quarterly, january 1889.

in addition to the idiom beginning with
month
also see:

by the day (month)
(for months) on end