This is the 4654th most frequent French word.
panini
A grilled or toasted sandwich, typically made with Italian bread.
Here, 'panini' represents the food item (singular) being ordered.
J'ai commandé un panini au jambon et fromage.
I ordered a ham and cheese panini.
In this case, 'panini' is used as the plural form of the food item.
Nous avons mangé des panini délicieux hier soir.
We had delicious paninis last night.
Again using 'panini' in the singular to refer to a specific preparation of the food.