[al-i-gey-nee] 
| a river flowing NW from Pennsylvania into SW New York and then S through W Pennsylvania, joining the Monongahela at Pittsburgh to form the Ohio River. 325 mi. (525 km) long. |
| a mountain range in Pennsylvania, Maryland, West Virginia, and Virginia: a part of the Appalachian Mountains. |
Allegheny
county, southwestern Pennsylvania, U.S., consisting of a hilly region on the Allegheny Plateau bounded to the southeast by the Monongahela and Youghiogheny rivers and to the northeast by the Allegheny River. The Ohio, Allegheny, and Monongahela rivers converge in the centre of the county to form an area known as the Golden Triangle; this was a strategic point of contention between the French and the English, who fortified the area with Fort Duquesne (1754) and Fort Pitt (1761), respectively. With the defeat of Pontiac's warriors (1763), the area opened up to settlers who founded Pittsburgh (1764), which became the county seat when Allegheny county was formed in 1788. The county's name is derived from the Delaware Indian word oolikhanna, meaning "good river." The Triangle is now Pittsburgh's central business district and the location of the popular Point State Park
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