When we lived in Umbria, crostini inevitably meant little slices of pane di Terni, smeared with a paté of veal spleen (milza) or chicken livers (crostini di fegatini or crostini neri), blended with capers and anchovies. Tuscans make crostini di cavalo nero with sautée curly kale and garlic. Since then they have become the tapas or pintxos of Italy.
Use your imagination, and put on whatever you like. Big ones are crostoni, which can also mean pieces of toasted bread rubbed with garlic and drizzled with olive oil, and placed in the bottom of a bowl before topping with soup, or served alongside.
Crostini also means croutons or rusks.
Image by Raul