Pepperoni is a misspell of peperoni, the Italian plural of peperone (literally "large pepper"). As an English word, it is used as a singular (plural form identical). In Italian the word refers to a large, strong-tasted fruit. Ordering pepperoni pizza is a nasty surprise for many American tourists.
In continental Europe, pepperoni are smallish peppers of low to medium hot taste marinated in salt-vinegar solution. It is usually the ripe fruit of sweet pepper (Capsicum Annuum).