The Collaborative International Dictionary of English v.0.48:

Impotence \Im"po*tence\, Impotency \Im"po*ten*cy\, n. [L.
   impotenia inability, poverty, lack of moderation. See
   Impotent.]
   1. The quality or condition of being impotent; lack of
      strength or power, animal, intellectual, or moral;
      weakness; feebleness; inability; imbecility.
      [1913 Webster]

            Some were poor by impotency of nature; as young
            fatherless children, old decrepit persons, idiots,
            and cripples.                         --Hayward.
      [1913 Webster]

            O, impotence of mind in body strong!  --Milton.
      [1913 Webster]

   2. Lack of self-restraint or self-control. [R.] --Milton.
      [1913 Webster]

   3. (Law & Med.) Lack of procreative power; inability to
      copulate, or beget children; also, sometimes, sterility;
      barrenness; specifically, in males: the inability to
      achieve or sustain a penile erection; erectile
      dysfunction.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

impotency
    n 1: the quality of lacking strength or power; being weak and
         feeble [syn: powerlessness, impotence, impotency]
         [ant: power, powerfulness]
    2: an inability (usually of the male animal) to copulate [syn:
       impotence, impotency] [ant: potence, potency]