Tympanon is the Italian for the English tympanum, but the two words can differ in meaning in a way which is confusing.
In English, following the vocabulary of the original Classical architects like Vitruvius, a pediment is a triangular element below the gabled roofline of a building, usually above the entablature on a facade. They were common elements in Classical architecture, and are found on many churches in Rome.
In later Roman architecture especially, little pediments were placed over doors, niches and windows as decorative features and this practice was imitated in Renaissance and Baroque architecture. Especially in the latter, pediments can be of other shapes such as segments or polygons. Further, they can be broken or split and the breaks curlicued.
A tympanum in English is the area enclosed by a pediment. It is traditionally decorated with sculpture, relief, mosaic or paintings. The word is also used to refer to the semi-circular surface created when an arch is filled, and these are common over doorways in Romanesque churches.
To summarize, in English a pediment contains a tympanum. However, in Italian the word tympanon can just means a pediment together with the surface it contains (tympanon = pediment + tympanum). This is despite the word frontone having the meaning of the English word pediment, so the Italians can distinguish the two English words if they want to.