pro·gres·sion /prəˈgrɛʃən/
 前進,連續,級數
 progression
 級數
 Pro·gres·sion n.
 1. The act of moving forward; a proceeding in a course; motion onward.
 2. Course; passage; lapse or process of time.
    I hope, in a short progression, you will be wholly immerged in the delices and joys of religion.   --Evelyn.
 3. Math. Regular or proportional advance in increase or decrease of numbers; continued proportion, arithmetical, geometrical, or harmonic.
 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.
 Arithmetical progression, a progression in which the terms increase or decrease by equal differences, as the numbers 2, 4, 6, 8, 1010, 8, 6, 4, 2by the difference 2.
 Geometrical progression, a progression in which the terms increase or decrease by equal ratios, as the numbers 2, 4, 8, 16, 32, 6464, 32, 16, 8, 4, 2by a continual multiplication or division by 2.
 -- Harmonic progression, a progression in which the terms are the reciprocals of quantities in arithmetical progression, as ½, ¼, ⅙, ⅛, ¹/₁₀.
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From: WordNet (r) 2.0

 progression
      n 1: a series with a definite pattern of advance [syn: patterned
           advance]
      2: a movement forward; "he listened for the progress of the
         troops" [syn: progress, advance]
      3: the act of moving forward toward a goal [syn: progress, procession,
          advance, advancement, forward motion, onward
         motion]