The Italian indefinite article (larticolo indeterminativo) corresponds to English a/an and is used with singular nouns. It also corresponds to the number one.

INDEFINITE ARTICLES
MASCHILE	FEMMINILE
uno zio (uncle)	una zia (aunt)
un cugino (cousin, m.)	una cugina (cousin, f.)
un amico (friend, m.)	un’amica (friend, f.)

Uno is used for masculine words beginning with z or s consonant. un is used for all other masculine words. Una is used for feminine words beginning with a consonant. un’ is used for feminine words beginning with a vowel.

un treno e una bicicletta
　　un aeroplano e un’automobile
　　uno stadio e una stazione