The Collaborative International Dictionary of English v.0.48:

Attenuate \At*ten"u*ate\, Attenuated \At*ten"u*a`ted\, a. [L.
   attenuatus, p. p.]
   1. Made thin or slender.
      [1913 Webster]

   2. Made thin or less viscid; rarefied. --Bacon.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Attenuate \At*ten"u*ate\, v. t. [imp. & p. p. Attenuated; p.
   pr. & vb. n. Attenuating.] [L. attenuatus, p. p. of
   attenuare; ad + tenuare to make thin, tenuis thin. See
   Thin.]
   1. To make thin or slender, as by mechanical or chemical
      action upon inanimate objects, or by the effects of
      starvation, disease, etc., upon living bodies.
      [1913 Webster]

   2. To make thin or less consistent; to render less viscid or
      dense; to rarefy. Specifically: To subtilize, as the
      humors of the body, or to break them into finer parts.
      [1913 Webster]

   3. To lessen the amount, force, or value of; to make less
      complex; to weaken.
      [1913 Webster]

            To undersell our rivals . . . has led the
            manufacturer to . . . attenuate his processes, in
            the allotment of tasks, to an extreme point. --I.
                                                  Taylor.
      [1913 Webster]

            We may reject and reject till we attenuate history
            into sapless meagerness.              --Sir F.
                                                  Palgrave.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

decreased \decreased\ adj.
   made less in size or amount or degree. Opposite of
   increased. [Narrower terms: attenuate, attenuated, faded,
   weakened; belittled, diminished, small; cut, cut-rate;
   diminished, lessened; minimized; remittent;
   attenuated]

   Syn: reduced.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

attenuated
    adj 1: of an electrical signal; reduced in amplitude with little
           or no distortion
    2: reduced in strength; "the faded tones of an old recording"
       [syn: attenuate, attenuated, faded, weakened]

Moby Thesaurus II by Grady Ward, 1.0:

145 Moby Thesaurus words for "attenuated":
   Sanforized, abated, ablated, adulterated, airy, attenuate, bated,
   belittled, boyish, cadaverous, consumed, contracted, corky,
   corpselike, curtailed, cut, dainty, decreased, deflated, delicate,
   diaphanous, dilute, diluted, diminished, dissipated, downy,
   dried-up, dropped, emacerated, emaciate, emaciated, eroded,
   ethereal, fallen, filmy, fine, fine-drawn, fine-grained, finespun,
   flimsy, fluffy, frail, fuzzy, gaseous, gauzy, girlish, gossamer,
   gossamery, gracile, haggard, hollow-eyed, insubstantial, jejune,
   lacy, less, lesser, light, lower, lowered, marantic, marasmic,
   miniaturized, misty, papery, parched, peaked, peaky, pinched, poor,
   preshrunk, pubescent, puny, rare, rarefied, reduced, refined,
   retrenched, satin, satiny, scaled-down, sear, shorn, shorter,
   shriveled, shriveled up, shrunk, shrunken, silky, skeletal,
   slender, slenderish, slight, slight-made, slim, slimmish, slinky,
   small, smaller, smooth, starved, starveling, subtile, subtle,
   svelte, sylphlike, tabetic, tabid, tenuous, thin, thin-bodied,
   thin-set, thin-spun, thinned, thinned-out, thinnish, threadlike,
   uncompact, uncompressed, underfed, undernourished, unsubstantial,
   vague, vaporous, velutinous, velvety, wasp-waisted, wasted,
   wasted away, watered, watered-down, watery, weak, weakened,
   weazened, weazeny, willowy, windy, wiredrawn, wispy, withered,
   wizen, wizen-faced, wizened, worn, wraithlike