The Collaborative International Dictionary of English v.0.48:

Sidereal \Si*de"re*al\, a. [L. sidereus, from sidus, sideris, a
   constellation, a star. Cf. Sideral, Consider, Desire.]
   1. Relating to the stars; starry; astral; as, sidereal
      astronomy.
      [1913 Webster]

   2. (Astron.) Measuring by the apparent motion of the stars;
      designated, marked out, or accompanied, by a return to the
      same position in respect to the stars; as, the sidereal
      revolution of a planet; a sidereal day.
      [1913 Webster]

   Sidereal clock, day, month, year. See under Clock,
      Day, etc.

   Sideral time, time as reckoned by sideral days, or, taking
      the sidereal day as the unit, the time elapsed since a
      transit of the vernal equinox, reckoned in parts of a
      sidereal day. This is, strictly, apparent sidereal time,
      mean sidereal time being reckoned from the transit, not of
      the true, but of the mean, equinoctial point.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Month \Month\ (m[u^]nth), n. [OE. month, moneth, AS.
   m[=o]n[eth], m[=o]na[eth]; akin to m[=o]na moon, and to D.
   maand month, G. monat, OHG. m[=a]n[=o]d, Icel. m[=a]nu[eth]r,
   m[=a]na[eth]r, Goth. m[=e]n[=o][thorn]s. [root]272. See
   Moon.]
   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.
   [1913 Webster]

   Note: In the common law, a month is a lunar month, or
         twenty-eight days, unless otherwise expressed.
         --Blackstone. In the United States the rule of the
         common law is generally changed, and a month is
         declared to mean a calendar month. --Cooley's
         Blackstone.
         [1913 Webster]

   A month mind.
   (a) A strong or abnormal desire. [Obs.] --Shak.
   (b) A celebration made in remembrance of a deceased person a
       month after death. --Strype.

   Calendar months, 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.

   Lunar month, 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.

   Solar month, 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

month
    n 1: one of the twelve divisions of the calendar year; "he paid
         the bill last month" [syn: calendar month, month]
    2: a time unit of approximately 30 days; "he was given a month
       to pay the bill"

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "month":
   abundant year, academic year, annum, bissextile year,
   calendar month, calendar year, century, common year, day, decade,
   decennary, decennium, defective year, fiscal year, fortnight, hour,
   leap year, lunar month, lunar year, lunation, luster, lustrum,
   man-hour, microsecond, millennium, millisecond, minute, moment,
   moon, quarter, quinquennium, regular year, second, semester,
   session, sidereal year, solar year, sun, term, trimester,
   twelvemonth, week, weekday, year



Easton's 1897 Bible Dictionary:

Month
   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 institution 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.


Bouvier's Law Dictionary, Revised 6th Ed (1856):

MONTH. A space of time variously computed, as it is applied to astronomical,
civil or solar, or lunar months.
     2. The astronomical month contains one-twelfth part of the time
employed by the sun in going through the zodiac. In law, when a month simply
is mentioned, it is never understood to mean an astronomical month.
     3. The civil or solar month is that which agrees with the Gregorian
calendar, and these months are known by the names of January, February,
March, &c. They are composed of unequal portions of time. There are seven of
thirty-one days each, four of thirty, and one which is sometimes composed of
twenty-eight days, and in leap years, of twenty-nine.
     4. The lunar mouth is composed of twenty-eight days only. When a law is
passed or contract made, and the month is expressly stated to be solar or
civil, which is expressed by the term calendar month, or when it is
expressed to be a lunar month, no difficulty can arise; but when time is
given for the performance of an act, and the word month simply is used, so
that the intention of the parties cannot be ascertained then the question
arises, how shall the month be computed? By the law of England a month means
ordinarily, in common contracts, as, in leases, a lunar month; a contract,
therefore, made for a lease of land for twelve months, would mean a lease
for forty-eight weeks only. 2 Bl. Com. 141; 6 Co. R. 62; 6 T. R. 224. A
distinction has been made between "twelve months," and "a twelve-month;" the
latter has been held to mean a year. 6 Co. R. 61.
     5. Among the Greeks and Romans the months were lunar, and probably the
mode of computation adopted in the English law has been adopted from the
codes of these countries. Clef des Lois Rom. mot Mois.
     6. But in mercantile contracts, a month simply signifies a calendar
month; a promissory note to pay money in twelve months, would therefore mean
a promise to pay in one year, or twelve calendar months. Chit. on Bills,
406; 1 John. Cas. 99; 3 B. & B. 187; 1 M. & S. 111; Story on Bills, Sec.
143; Story, P. N. Sec. 213; Bayl. on Bills, c. 7; 4 Kent, Comm. Sect. 56; 2
Mass. 170; 4 Mass. 460; 6 Watts. & Serg. 179.
     7. In general, when a statute Speaks of a month, without adding
"calendar," or other words showing a clear intention, it shall be intended a
lunar month. Com. Dig. Ann. B; 4 Wend. 512; 15 John. R. 358. See 2 Cowen, R.
518; Id. 605. In all legal proceedings, as in commitments, pleadings, &c. a
month means four weeks. 3 Burr. R. 1455; 1 Bl. Rep. 450; Dougl. R. 446 463.
     8. In Pennsylvania and Massachusetts, and perhaps some other states, 1
Hill. Ab. 118, n., a month mentioned generally in a statute, has been
construed to mean a calendar month. 2 Dall. R. 302; 4 Dall. Rep. 143; 4
Mass. R. 461; 4 Bibb. R. 105. In England, in the ecclesiastical law, months
are computed by the calendar. 3 Burr. R. 1455; 1 M. & S. 111.
     9. In New York, it is enacted that whenever the term "month," or
"months," is or shall be used in any statute, act, deed, verbal or written
contract, or any public or private instrument whatever, it shall be
construed to mean a calendar, and not a lunar month; unless otherwise
expressed. Rev. Stat. part 1, c. 19, tit. 1, Sec. 4. Vide, generally, 2 Sim.
& Stu. 476; 2 A. K. Marsh. Rep. 245; 3 John. Ch. Rep. 74; 2 Campb. 294; 1
Esp. R. 146; 6 T. R. 224; 1 M. & S. 111; 3 East, R. 407; 4 Moore, 465; 1 Bl.
Rep. 150; 1 Bing. 307; S. C. 8 Eng. C. L. R. 328;. 1 M. & S. 111; 1 Str.
652; 6 M. & S. 227; 3 Brod. & B. 187; S. C. 7 Eng. C. L. R. 404.