The Collaborative International Dictionary of English v.0.48:

Oppression \Op*pres"sion\, n. [F., fr. L. oppressio.]
   1. The act of oppressing, or state of being oppressed.
      [1913 Webster]

   2. That which oppresses; a hardship or injustice; cruelty;
      severity; tyranny. "The multitude of oppressions." --Job
      xxxv. 9.
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   3. A sense of heaviness or obstruction in the body or mind;
      depression; dullness; lassitude; as, an oppression of
      spirits; an oppression of the lungs.
      [1913 Webster]

            There gentle Sleep
            First found me, and with soft oppression seized
            My drowsed sense.                     --Milton.
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   4. Ravishment; rape. [Obs.] --Chaucer.
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WordNet (r) 3.0 (2006):

oppression
    n 1: the act of subjugating by cruelty; "the tyrant's oppression
         of the people" [syn: oppression, subjugation]
    2: the state of being kept down by unjust use of force or
       authority: "after years of oppression they finally revolted"
    3: a feeling of being oppressed [syn: oppression,
       oppressiveness]