The Collaborative International Dictionary of English v.0.48:

Prime \Prime\, a. [F., fr. L. primus first, a superl.
   corresponding to the compar. prior former. See Prior, a.,
   Foremost, Former, and cf. Prim, a., Primary,
   Prince.]
   1. First in order of time; original; primeval; primitive;
      primary. "Prime forests." --Tennyson.
      [1913 Webster]

            She was not the prime cause, but I myself. --Milton.
      [1913 Webster]

   Note: In this sense the word is nearly superseded by
         primitive, except in the phrase prime cost.
         [1913 Webster]

   2. First in rank, degree, dignity, authority, or importance;
      as, prime minister. "Prime virtues." --Dryden.
      [1913 Webster]

   3. First in excellence; of highest quality; as, prime wheat;
      a prime quality of cloth.
      [1913 Webster]

   4. Early; blooming; being in the first stage. [Poetic]
      [1913 Webster]

            His starry helm, unbuckled, showed him prime
            In manhood where youth ended.         --Milton.
      [1913 Webster]

   5. Lecherous; lustful; lewd. [Obs.] --Shak.
      [1913 Webster]

   6. Marked or distinguished by a mark (') called a prime mark.

   Note: In this dictionary the same typographic mark is used to
         indicate a weak accent in headwords, and minutes of a
         degree in angle measurements.
         [1913 Webster]

   7. (Math.)
      (a) Divisible by no number except itself or unity; as, 7
          is a prime number.
      (b) Having no common factor; -- used with to; as, 12 is
          prime to 25.
          [Webster 1913 Suppl.]

   Prime and ultimate ratio. (Math.). See Ultimate.

   Prime conductor. (Elec.) See under Conductor.

   Prime factor (Arith.), a factor which is a prime number.

   Prime figure (Geom.), a figure which can not be divided
      into any other figure more simple than itself, as a
      triangle, a pyramid, etc.

   Prime meridian (Astron.), the meridian from which longitude
      is reckoned, as the meridian of Greenwich or Washington.


   Prime minister, the responsible head of a ministry or
      executive government; applied particularly to that of
      England.

   Prime mover. (Mech.)
      (a) A natural agency applied by man to the production of
          power. Especially: Muscular force; the weight and
          motion of fluids, as water and air; heat obtained by
          chemical combination, and applied to produce changes
          in the volume and pressure of steam, air, or other
          fluids; and electricity, obtained by chemical action,
          and applied to produce alternation of magnetic force.
      (b) An engine, or machine, the object of which is to
          receive and modify force and motion as supplied by
          some natural source, and apply them to drive other
          machines; as a water wheel, a water-pressure engine, a
          steam engine, a hot-air engine, etc.
      (c) Fig.: The original or the most effective force in any
          undertaking or work; as, Clarkson was the prime mover
          in English antislavery agitation.

   Prime number (Arith.), a number which is exactly divisible
      by no number except itself or unity, as 5, 7, 11.

   Prime vertical (Astron.), the vertical circle which passes
      through the east and west points of the horizon.

   Prime-vertical dial, a dial in which the shadow is
      projected on the plane of the prime vertical.

   Prime-vertical transit instrument, a transit instrument the
      telescope of which revolves in the plane of the prime
      vertical, -- used for observing the transit of stars over
      this circle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prime factor
    n 1: the prime factors of a quantity are all of the prime
         quantities that will exactly divide the given quantity