"Semis" or "semissis" was originally a Roman unit of weight equal to 1/2 of an "as" (thus a semi-as, from whence semis is possibly derived), which was the Roman pound (libra) of about 327.45 grams. A semis was equal to six unciae (ounces), or 163.73 grams, since the Roman pound was comprised of twelve unciae. The terms "semis" and "semissis" are actually interchangeable, but convention has been to assign the term "semis" to the bronze half-as, while "semissis" is generally applied to the late Roman golden half-solidus (see Semissis).
When the first Roman bronze coinage was produced in the first half of the third century BC, it was made on the literal weight standard from the "as" down to the semuncia, or half-ounce. Thus an "as" literally weighed one Roman pound, and the semis weighed half of a pound. The mark of value put on the semis was an "S". Subsequent revisions of the coinage system finally brought the weight standard down to the "semuncial" standard by about 90 BC. The "semuncial" standard meant that the unit (or "as") was struck at a literal weight of one-half of an uncia, or about 13.6 grams, with the other denominations being proportionately reduced. Hence the semis, instead of being half of a pound, came to be struck at a quarter of a Roman ounce, or about 6.8 grams. However in practice the semis was rarely struck after about 90 BC, although it was revived by Mark Antony.
Augustus did not strike the semis until the final five years of his reign. Very scarce issues from Lugdunum in orichalcum exist for both he and his heir Tiberius. Thereafter the semis was not struck until about 62 by Nero, who struck them at about 6.35 grams of copper. About 64 Nero experimented by making the semisses of orichalcum at a reduced weight of about 3.65 grams, but quickly abandoned the experiment by returning the semisses to copper in 65, and ceased their production entirely by the end of 66. Part of Nero's experiment was also to restore the old mark of value "S" to the semisses, but that also ended by 65. After Nero the semis was not struck again until Vespasian, and only sparingly thereafter. In the mid-third century the traditionalist Trajan Decius attempted to revive the semis, but after his brief two-year reign the semis became very rare (as did all copper and bronze due to the effect of the debasement of the antoninianus). The last semis was issued by Diocletian before his coinage reform of 294. The last semis, issued at about 5.4 grams, was still close to the original imperial weight standard. Its diameter, about 22 millimeters, was within the typical range of 18-22 mms of the earlier pieces.
The uniform obverse of the semis was the laureate bust, and since it was half of an "as" it was valued at 1/32 of a denarius.