The semis (“SEH-miss”) coin was worth half of an as, and was first made in 280 BC. After the Roman Republic came to an end, the semis was continued into the Roman Empire. However, it was never a regularly-issued coin, and the production of semisses ended with the reign of Hadrian (117 — 138 AD).
Note that the translation for semis in Italian is semisse; look for that term in sales listings which may include Italian-speaking sellers.