The term piranha is borrowed from the Tupi-Guarani languages which are spoken by the indigenous people in South America.

First attested in 1869. The first part of the term is from Old Tupi pirá (“fish”). There are several theories regarding the source of second part, including:

1. One is from Old Tupi sanha, sainha (“tooth”), or a Guaraní cognate of the same.

2. Another is from Old Tupi ánha, anhá (“cut”) (which also meant "bad" or "devil" in Tupi-Guarani).

The full term may have been 'pira nya', a variant of 'pira'ya' (“scissor-fish”), in Old Tupi. The term entered English via Portuguese (as piranha) and Spanish (as piraña).

Piranhas are indigenous to the Amazon basin, in the Orinoco, in rivers of the Guianas, in the Paraguay–Paraná, and the São Francisco River systems, but there are major differences in the species richness. In a review where 38–39 piranha species were recognized, 25 were from the Amazon and 16 from Orinoco, while only three were present in Paraguay–Paraná and two in São Francisco. Most species are restricted to a single river system, but some (such as the red-bellied piranha) occur in several. Many species can occur together; for example, seven are found in Caño Maporal, a stream in Venezuela.

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