The panino is the base term itself, and what defines it is a discipline rather than a recipe: the Italian sandwich names one thing and gets out of its way. Where other countries stack, the panino is built on subtraction. A few slices of a single cured meat, or one cheese, or one cooked vegetable, set on the right bread, with the restraint to add nothing that would mask it. Prosciutto and bread. Mortadella and bread. A wedge of pecorino and bread. The whole idea is that a single ingredient chosen at its peak does not need company, and the sandwich exists to frame it cleanly, not to bury it under a second and third voice.
The craft is the bread and the cut, because with so little inside, both decisions carry the sandwich. The bread is usually a crusty roll or a length of crisp-shelled loaf, chosen so it holds its structure against the filling rather than collapsing into it, and matched regionally to what goes inside: a hollow rosetta filled light, a chewy ciabatta taking an oil-dressed filling, a dense country loaf carrying a strong cured meat. The filling is cut to its character, a fatty prosciutto shaved paper-thin and laid in loose folds so air moves through it, a firm cheese cut thick to keep its bite. Butter or oil appears rarely and only where it bridges a very lean filling to the crust. The quality is decided at the bakery and the norcineria, not at the cutting board.
The variations are the whole map of Italy, because almost every Italian sandwich is a panino named for its filling, its bread, or its town: the salume panino and the formaggio panino that are the two halves of the deli counter, the regional flatbreads, the offal rolls, the place-named city panini. Each is one larder on one local bread, and each deserves its own article rather than being crowded in here, which is what the rest of this catalog is for.