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