The Collaborative International Dictionary of English v.0.48:

Progression \Pro*gres"sion\, n. [L. progressio: cf. F.
   progression.]
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   1. The act of moving forward; a proceeding in a course;
      motion onward.
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   2. Course; passage; lapse or process of time.
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            I hope, in a short progression, you will be wholly
            immerged in the delices and joys of religion.
                                                  --Evelyn.
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   3. (Math.) Regular or proportional advance in increase or
      decrease of numbers; continued proportion, arithmetical,
      geometrical, or harmonic.
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   4. (Mus.) A regular succession of tones or chords; the
      movement of the parts in harmony; the order of the
      modulations in a piece from key to key.
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   Arithmetical progression, a progression in which the terms
      increase or decrease by equal differences, as the numbers
      [lbrace2]2, 4, 6, 8, 1010, 8, 6, 4, 2[rbrace2] by the
      difference 2.
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   Geometrical progression, a progression in which the terms
      increase or decrease by equal ratios, as the numbers
      [lbrace2]2, 4, 8, 16, 32, 6464, 32, 16, 8, 4, 2[rbrace2]
      by a continual multiplication or division by 2.
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   Harmonic progression, a progression in which the terms are
      the reciprocals of quantities in arithmetical progression,
      as 1/2, 1/4, 1/6, 1/8, 1/10.
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The Collaborative International Dictionary of English v.0.48:

Arithmetical \Ar`ith*met"ic*al\, a.
   Of or pertaining to arithmetic; according to the rules or
   method of arithmetic.
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   Arithmetical complement of a logarithm. See Logarithm.

   Arithmetical mean. See Mean.

   Arithmetical progression. See Progression.

   Arithmetical proportion. See Proportion.
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