Because the Italians like to keep things interesting (read: confusing), this year’s election will take place with a never before tested electoral law (legge elettorale). It’s called ‘Rosatellum‘, taking its name from Ettore Rosato, who leads the Democratic Party in the Lower House and drafted an early version of the law. This replaced an old electoral law, which was called ‘Italicum‘, and was only valid for the Chamber of Deputies.
The system is anything but simple: it’s a mixed voting system with some seats allocated proportionally (using ‘un sistema proporzionale‘) and the others using first-past-the-post (uninominale secco). Another key bit of vocab is the ‘soglia di sbarramento‘ or ‘election threshold’: the minimum share of the vote every party and coalition must achieve in order to get any seats.
Source: The political Italian vocabulary you should know ahead of the 2018 election