In Italian, there are two types of invariable nouns: invariable by gender and invariable by number. When we say “by number,” we mean that they have the same form whether they are singular or plural.
Here are some examples:
Masculine nouns that end in -a:
il sosia (i sosia), la sosia (le sosia) look-alike
Feminine nouns that end in -o:
l’eco (f.) echo (the masculine version of the noun has two forms: l’eco, gli echi)
When the nouns have one syllabus, they are sometimes invariable:
il re (i re) king
lo sci (gli sci) ski (plural = un paio di sci / a pair of skis)
Feminine nouns that end in -ie (except for moglie, superficie, effigie):
la calvizie baldness
la serie series
la progenie offspring
Some colors, when used as nouns, are also invariable and masculine:
blu
rosa
viola
Truncated nouns are also invariable:
la bici, le bici bicycle
la moto, le moto motorcycle
Nouns that end in consonants are invariable:
il film film, movie
il bar cafè