Source : Webster's Revised Unabridged Dictionary (1913)

Allegheny \Al"le*ghe`ny\, a.
   1. Of or pertaining to the Allegheny Mountains, or the region
      where they are situated. Also

Appalachian \Ap`pa*la"chi*an\, a.
   Of or pertaining to a chain of mountains in the United
   States, commonly called the {Allegheny} mountains.

   Note: The name Appalachian was given to the mountains by the
         Spaniards under De Soto, who derived it from the
         neighboring Indians. --Am. Cyc.

Source : WordNet®

Allegheny
     n : a river that rises in Pennsylvania and flows north into New
         York and then back south through Pennsylvania again to
         join the Monongahela River at Pittsburgh which is the
         beginning of the Ohio River [syn: {Allegheny River}]