In grammar, the word singular denotes a quantity of one, in contrast with plural, which means more than one. For example:
un libro one book or a book
the word libro is singular, it is a singular noun
la caja the box
caja is also a singular noun
Plural forms of these nouns would be libros (books) and cajas (boxes), as in the following examples:
dos libros, cien libros, muchos libros two books, one hundred books, many books
tres cajas, algunas cajas, varias cajas three boxes, some boxes, several boxes
The terms singular and plural are values of the grammatical category of number. Number affects nouns, pronouns, adjectives and verbs. Here are some simple examples of singular pronouns, adjectives and verbs:
Ella habla. She speaks.
Ella is a singular pronoun and habla is a singular verb.
Esa mujer ha cogido mi revista. That woman has taken my magazine.
Esa is a singular demonstrative adjective, ha cogido is a singular verb, mi is a singular possessive adjective and mujer and revista are singular nouns.
Fue solo un mal sueño. It was only a bad dream.
Fue is a singular verb, mal is a singular adjective and sueño is a singular noun.