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"La foca" is a form of "foca", a noun which is often translated as "seal". "El sello" is a form of "sello", a noun which is often translated as "stamp". Learn more about the difference between "la foca" and "el sello" below.
la foca(
foh
-
kah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (zoology)
a. seal 
Las focas podrán parecer torpes en tierra, pero son increíblemente ágiles en el mar.Seals might look clumsy on land, but they are incredibly agile in the ocean.
Seis tipos de focas viven en la Antártida.Six types of seals live in Antarctica.
2. (material)
3. (colloquial) (fat person)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
a. fatty  (colloquial)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Sigue comiendo así y te pondrás como una foca.Keep eating like that and you'll turn into a fatty.
el sello(
seh
-
yoh
)A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
1. (mailing)
a. stamp 
Esta carta requiere que pongas tres sellos por su peso.This letter requires three stamps because of its weight.
2. (label)
a. rubber stamp  (tool)
Al fin ha llegado el sello para estampar las cartas oficiales.The rubber stamp has finally arrived to stamp the official letters.
c. seal  (wax seal)
Siempre cierro mis cartas con un sello que lleva mis iniciales.I always seal my letters with a seal bearing my initials.
3. (publisher)
a. record label  (music)
El cantante estaba emocionado por su reunión con el sello famoso.The singer was very excited about his meeting with the famous record label.
4. (logo)
a. brand 
El mundo entero reconoce el sello de McDonald's.The whole world recognizes the McDonald's brand.