The Collaborative International Dictionary of English v.0.48:

Series \Se"ries\, n. [L. series, fr. serere, sertum, to join or
   bind together; cf. Gr. ??? to fasten, Skr. sarit thread. Cf.
   Assert, Desert a solitude, Exert, Insert,
   Seraglio.]
   1. A number of things or events standing or succeeding in
      order, and connected by a like relation; sequence; order;
      course; a succession of things; as, a continuous series of
      calamitous events.
      [1913 Webster]

            During some years his life a series of triumphs.
                                                  --Macaulay.
      [1913 Webster]

   2. (Biol.) Any comprehensive group of animals or plants
      including several subordinate related groups.
      [1913 Webster]

   Note: Sometimes a series includes several classes; sometimes
         only orders or families; in other cases only species.
         [1913 Webster]

   3. (Bot.) In Engler's system of plant classification, a group
      of families showing certain structural or morphological
      relationships. It corresponds to the cohort of some
      writers, and to the order of many modern systematists.
      [Webster 1913 Suppl.]

   4. (Math.) An indefinite number of terms succeeding one
      another, each of which is derived from one or more of the
      preceding by a fixed law, called the law of the series;
      as, an arithmetical series; a geometrical series.
      [1913 Webster]

   5. (Elec.) A mode of arranging the separate parts of a
      circuit by connecting them successively end to end to form
      a single path for the current; -- opposed to parallel.
      The parts so arranged are said to be

   in series.
      [Webster 1913 Suppl.]

   6. (Com.) A parcel of rough diamonds of assorted qualities.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Cohort \Co"hort\, n. [L. cohors, prop. an inclosure: cf. F.
   cohorte. See Court, n.]
   1. (Rom. Antiq.) A body of about five or six hundred
      soldiers; the tenth part of a legion.
      [1913 Webster]

   2. Any band or body of warriors.
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            With him the cohort bright
            Of watchful cherubim.                 --Milton.
      [1913 Webster]

   3. (Bot.) A natural group of orders of plants, less
      comprehensive than a class.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cohort
    n 1: a company of companions or supporters
    2: a band of warriors (originally a unit of a Roman Legion)
    3: a group of people having approximately the same age [syn:
       age group, age bracket, cohort]