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Medusa vs. água-viva vs. alforreca

Word practice demo
While medusa, água-viva, and alforreca all refer to nature's gelatinous sea creature known as the jellyfish, the choice depends heavily on geography. Água-viva is the standard term in Brazil, whereas medusa and alforreca are the dominant terms in Portugal.

Medusa

B1
This is the standard term used in European Portuguese (Portugal) and is also the universal scientific term used in biology across all regions. In Brazil, medusa is understood but sounds formal or technical compared to água-viva.
A medusa é um animal fascinante de observar no aquário.
(The jellyfish is a fascinating animal to observe at the aquarium.)
Cuidado para não tocarem nos tentáculos da medusa.
(Be careful not to touch the tentacles of the jellyfish.)
Esta espécie de medusa é perigosa para os humanos.
(This species of jellyfish is dangerous to humans.)

Água-viva

B1
Literally translating to living water, this is the everyday word used in Brazilian Portuguese. While it is understood in Portugal, it is rarely used there in daily conversation.
Minha filha tem medo de encontrar uma água-viva no mar.
(My daughter is afraid of finding a jellyfish in the sea.)
O salva-vidas avisou que a praia está cheia de água-viva.
(The lifeguard warned that the beach is full of jellyfish.)
Uma queimadura de água-viva arde bastante.
(A jellyfish sting burns quite a lot.)

Alforreca

B2
This term is specific to European Portuguese (Portugal) and is used as a common synonym for medusa. It is practically unknown or considered very obscure in Brazil.
Ontem vi uma alforreca morta na areia.
(Yesterday I saw a dead jellyfish on the sand.)
As redes dos pescadores vieram cheias de alforrecas.
(The fishermen's nets came up full of jellyfish.)
Dizem que as alforrecas aparecem quando a água está quente.
(They say that jellyfish appear when the water is warm.)

Summary

To sound natural, use água-viva if you are in Brazil. If you are in Portugal, you should use medusa or alforreca. While medusa is also the correct scientific term in both countries, alforreca is exclusively European.