Cordeiro vs. borrego
In Portuguese, both cordeiro and borrego translate to lamb, but they are not always interchangeable. The main differences relate to the animal's age, its purpose (especially culinary), and regional usage.
Cordeiro
A2Cordeiro is the most general and widely used term for a young sheep, typically up to one year old, and often refers to a suckling lamb (cordeiro de leite). It is also the standard term used in religious and figurative contexts.
O pastor guiava o seu rebanho e o pequeno cordeiro seguia a mãe.
(The shepherd guided his flock and the little lamb followed its mother.)
Para o almoço de Páscoa, a minha avó vai preparar um assado de cordeiro.
(For the Easter lunch, my grandmother is going to prepare a lamb roast.)
Na Bíblia, Jesus é frequentemente chamado de Cordeiro de Deus.
(In the Bible, Jesus is often called the Lamb of God.)
A criança era mansa como um cordeiro e não se queixava de nada.
(The child was as meek as a lamb and didn't complain about anything.)
Borrego
B1Borrego typically refers to a young sheep that has been weaned but is not yet fully grown, usually between six months and one year old. It is very common in agricultural and culinary contexts, especially in Portugal, where it is often the default term for lamb meat.
O ensopado de borrego é um prato tradicional da cozinha alentejana.
(Lamb stew is a traditional dish from Alentejo cuisine.)
O criador levou os seus melhores borregos para vender na feira agrícola.
(The breeder took his best young sheep to sell at the agricultural fair.)
No talho, pedi costeletas de borrego para o churrasco de domingo.
(At the butcher shop, I asked for lamb chops for Sunday's barbecue.)
A lã deste borrego é particularmente macia e valorizada.
(The wool from this young sheep is particularly soft and valued.)
Summary
In summary, cordeiro is the broader, more common term for a young sheep, especially a suckling one, and is used in religious contexts. Borrego is more specific, often referring to a slightly older, weaned lamb destined for meat, and is the preferred culinary term in many parts of Portugal. While a borrego is always a cordeiro, a very young cordeiro might not yet be called a borrego.