Lime vs. limetta
The Italian words lime and limetta both refer to the green citrus fruit known as lime. The main difference lies in their origin, grammar, and everyday usage. While one is a modern loanword used universally in daily life, the other is the traditional, native Italian term reserved for more specific contexts.
Lime
The word lime is an English loanword and is the most common, everyday term used in modern Italian to describe the fruit. It is a masculine noun (il lime) and is invariable, meaning the plural remains lime. You will hear this word constantly in supermarkets, bars, restaurants, and cooking recipes.
Ho comprato tre lime per preparare i cocktail di stasera.
(I bought three limes to prepare tonight's cocktails.)
Spremi il succo di mezzo lime sull'insalata di mare.
(Squeeze the juice of half a lime over the seafood salad.)
Il barista ha decorato il bicchiere con una fetta di lime.
(The bartender decorated the glass with a slice of lime.)
Preferisco usare il lime piuttosto che il limone per fare questa torta.
(I prefer to use lime rather than lemon to make this cake.)
I lime al mercato costano molto poco durante l'estate.
(Limes at the market cost very little during the summer.)
Limetta
The word limetta is the proper, native Italian term for the lime fruit and the lime tree. It is a feminine noun (la limetta) with a regular plural (le limette). Today, it sounds slightly formal or dated to most Italians and is almost exclusively used in botanical, agricultural, scientific, or perfumery contexts.
La limetta è un agrume che cresce molto bene nei climi tropicali.
(The lime is a citrus fruit that grows very well in tropical climates.)
Nel giardino botanico abbiamo ammirato un antico albero di limetta.
(In the botanical garden we admired an ancient lime tree.)
Questo profumo contiene note rinfrescanti di bergamotto e limetta.
(This perfume contains refreshing notes of bergamot and lime.)
La coltivazione della limetta richiede abbondante acqua e sole diretto.
(The cultivation of the lime requires abundant water and direct sun.)
L'olio essenziale di limetta ha forti proprietà purificanti per la pelle.
(Lime essential oil has strong purifying properties for the skin.)
Summary
In short, use lime for almost all everyday situations, such as grocery shopping, cooking, or ordering drinks, keeping in mind that it is a masculine, invariable noun. Use limetta when you are reading or speaking in a botanical, agricultural, or formal context where the native feminine Italian word is required.