The Collaborative International Dictionary of English v.0.48:

panther \pan"ther\ (p[a^]n"th[~e]r), n. [OE. pantere, F.
   panth[`e]re, L. panthera, Gr. pa`nqhr, prob. fr. Skr.
   pundr[imac]ka a tiger.]
   [1913 Webster]
   1. (Zool.) A large dark-colored variety of the leopard, by
      some Zoologists considered a distinct species. It is
      marked with large ringlike spots, the centers of which are
      darker than the color of the body.
      [1913 Webster]

   2. (Zool.) In America, the name is applied to the puma, or
      cougar, and sometimes to the jaguar.
      [1913 Webster]
      [1913 Webster]

   Panther cat (Zool.), the ocelot.

   Panther cowry (Zool.), a spotted East Indian cowry
      (Cypr[ae]a pantherina); -- so called from its color.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Jaguar \Ja*guar"\, n. [Braz. yago['a]ra: cf. & Pg. jaguar.]
   (Zool.)
   A large and powerful feline animal (Panthera onca, formerly
   Felis onca), ranging from Texas and Mexico to Patagonia. It
   is usually brownish yellow, with large, dark, somewhat
   angular rings, each generally inclosing one or two dark
   spots. It is chiefly arboreal in its habits. It is also
   called the panther and the American tiger.
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Puma \Pu"ma\ (p[=u]"m[.a]), n. [Peruv. puma.] (Zool.)
   A large American carnivore (Felis concolor), found from
   Canada to Patagonia, especially among the mountains. Its
   color is tawny, or brownish yellow, without spots or stripes.
   Called also catamount, cougar, American lion, mountain
   lion, and panther or painter.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cougar \Cou"gar\ (k??"g?r), n. [F. couguar, from the native name
   in the South American dialects, cuguacuara, cuguacuarana.]
   (Zool.)
   An American feline quadruped (Felis concolor), resembling
   the African panther in size and habits. Its color is tawny,
   without spots; hence writers often called it the American
   lion. Called also puma, panther, mountain lion, and
   catamount. See Puma.
   [1913 Webster]

WordNet (r) 3.0 (2006):

panther
    n 1: a large spotted feline of tropical America similar to the
         leopard; in some classifications considered a member of the
         genus Felis [syn: jaguar, panther, Panthera onca,
         Felis onca]
    2: a leopard in the black color phase
    3: large American feline resembling a lion [syn: cougar,
       puma, catamount, mountain lion, painter, panther,
       Felis concolor]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "panther":
   Leo, Siberian tiger, bobcat, cat-a-mountain, catamount, cheetah,
   cougar, jaguar, leopard, lion, lynx, mountain lion, ocelot,
   painter, puma, simba, tiger, wildcat