The Collaborative International Dictionary of English v.0.48:

Intrenchment \In*trench"ment\, n. [From Intrench.]
   [1913 Webster]
   1. The act of intrenching or the state of being intrenched.
      [1913 Webster]

   2. (Mil.) Any defensive work consisting of at least a trench
      or ditch and a parapet made from the earth thrown up in
      making such a ditch.
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            On our side, we have thrown up intrenchments on
            Winter and Prospect Hills.            --Washington.
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   3. Any defense or protection.
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   4. An encroachment or infringement.
      [1913 Webster]

            The slight intrenchment upon individual freedom.
                                                  --Southey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intrenchment
    n 1: an entrenched fortification; a position protected by
         trenches [syn: entrenchment, intrenchment]