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"A" is an indefinite article which is often translated as "un", and "What is that?" is a phrase which is often translated as "¿Qué es eso?". Learn more about the difference between "a" and "What is that?" below.
a(
ey
)An indefinite article is a word that indicates that a noun is not specific (e.g., a girl).
A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
A noun is a word referring to a person, animal, place, thing, feeling, or idea (e.g., man, dog, house).
3. (letter)
4. (grade)
a. el sobresaliente (M)
(m) means that a noun is masculine. Spanish nouns have a gender, which is either feminine (like la mujer or la luna) or masculine (like el hombre or el sol).
I got an A on all my exams.Saqué sobresaliente en todos los exámenes.
5. (music)
An abbreviation is the shortened form of a word or group of words (e.g., "UFO" = "unidentified flying object"; "p." = "page").
a. A 
How many N is 1 A?¿A cuántos N equivale 1 A?
The intensity of this electric current is 10 A.La intensidad de esta corriente eléctrica es de 10 A.
What is that?(  
wuht
ihz
thaht
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
a. ¿Qué es eso? 
"What is that?" she asked, pointing to a flashing light on the control panel."¿Qué es eso?" preguntó, señalando una luz intermitente en el tablero de control.