The Apennines or Apennine Mountains (/ˈæpənaɪn/; Greek: Ἀπέννινα Ὄρη, Latin: Appenninus or Apenninus Mons—a singular used in the plural;Italian: Appennini) are a mountain range consisting of parallel smaller chains extending c. 1,200 km (750 mi) along the length of peninsular Italy. In the northwest they join with the Ligurian Alps at Altare. In the southwest they end at Reggio di Calabria, the coastal city at the tip of the peninsula. Since about 2000 the Ministry of the Environment of Italy, following the recommendations of the Apennines Park of Europe Project, has been defining the Apennines System to include the mountains of north Sicily, for a total distance of 1,500 kilometres (930 mi). The system forms an arc enclosing the east side of the Ligurian and Tyrrhenian Seas.

The etymology most frequently repeated, because of its semantic appropriateness, is that it derives from the Celtic Penn, "mountain, summit":A-penn-inus, which could have been assigned during the Celtic domination of north Italy in the 4th century BC or before. The name originally applied to the north Apennines. However, historical linguists have never found a derivation with which they are universally comfortable. Wilhelm Deecke said:"...its etymology is doubtful but some derive it from the Ligurian-Celtish Pen or Ben, which means mountain peak."