For some Roman coins the ancient name of the denominations are unknown. Antoninianus is a modern name for the double denarius denomination (taken from the real name of Caracalla, who created the denomination).
Antoninianus (215 - 274)
The antoninianus is a Roman double denarius coin denomination (pl. antoniniani) struck from 215 to 293 A.D. (or 274 if you consider the later issues, sometimes called aurelianiani, a new denomination). On the obverse of the antoninianus the emperor is depicted wearing a radiate crown, caesars are bare-headed, and empresses are shown with a lunar crescent behind their shoulders. On some later antoniniani the emperor wears a helmet. The ancient name for the type is unknown. Our modern name for it, antoninianus, names it after Marcus Aurelius Antoninus (nicknamed Caracalla) who introduced it in A.D. 215. Although the antoninianus was valued at two denarii, the weight was considerably less than that of two denarii. During the reign of Gordian III the antoninianus replaced the denarius as the primary Roman denomination. The first antoniniani were first struck with an alloy containing about 49.5% silver, but the denomination was continually debased until by 274 the alloy contained only 2.5% silver. In 274, Aurelian reformed the radiate denomination, striking on a copper core with a 5% silver coating, and valuing the new coin at 5 denarii commune. Older antoniniani no longer circulated after this reform and the radiate coins struck after the reform of 274 until 293 are also called aurelianianus. The antoninianus (or aurelianianus) was used until reform of Diocletian in 293.
Aurelianianus (274 - 293 A.D.)
The aurelianianus was a silver clad radiate denomination introduced by Aurelian in 274 A.D. and struck until 293 A.D. After 274 the older radiate Antoniniani no longer circulated. The aurelianianus was struck on the same standard 3.88 grams, with a copper core and a 5% silver coating, and tariffed at 5 denarii communes (d.c.). Many aurelianianus have lost their silvering and appear bronze today. Many of the later aurelianianus were struck with value marks, XXI or KA (Greek 20), indicating a value of 20 sestertii = 1 aurelianianus. Most references and dealer catalogs do not distinguish the aurelianianus from the earlier radiate denomination the antoninianus. Both terms, antoninianus and aurelianianus are modern terms. The names used by the Romans for the denominations are unknown.
In 293, Diocletian introduced the follis (plural: folles) a new larger denomination with a laureate portrait to replace the radiate antoninianus (or aurelianianus). Diocletian's new denomination had a copper core, and a 5% silver plate, the same as the radiates it replaced, but was c. 28 - 32 mm diameter, on a consistent weight standard of 10.75 grams. The follis was initially tariffed at 5 denarii communes (also the same as the radiates it replaced), but was later devalued to 12.5 d.c. and then to 25 d.c. Smaller fraction were struck with a radiate portrait, called "post-reform radiates." On the tetrarchic folles, portraits of the various emperors were highly stylized and usually indistinguishable. Often the emperor depicted can only be determined by the obverse legend. The most common reverse type had the legend GENIO POPVLI ROMANI, and depicted the Genius (spirit) of the Roman people standing making a sacrifice. In 301, the western mints struck a common type with the legend Sacra Moneta, and depicting Moneta standing holding scales and a cornucopia. The large folles of the Tetrarchy fell out of circulation in 306 when weight standards were reduced.
Late Roman Empire Denominations
In 307, Constantine introduced a reduced size and weight silver plated follis (nummis). Over the fourth and fifth centuries the nummis was continually reduced and also debased until the silver plating disappeared. Different denominations were issued but we often do not know what they were called by the Romans and often different size coins were the same denomination, just again reduced. We don't know with certainty the ancient names or values of many of the late Roman bronze coins and refer to them with designations based on size.
AE1 = c. 26 mm - 30 mm. Sometimes called the maiorina, sometimes a double-centenionalis.
AE2 = c. 20 mm - 26 mm. Sometimes called a centenionalis. At first it was a coin about 22 mm - 26 mm and weighing 4.9 - 6.1 grams, but later--the "reduced centenionalis" was 20 - 22 mm and about 3.0 to 4.7 grams.
AE4 = less than 17mm. Properly called nummus minimus, but also called a nummus, a small coin, heavily leaded, and devoid of silver. Initially these coins were 13-16 mm and 1.3 - 1.7 grams but declined in the late 4th and 5th centuries to where they are only 9.5 - 12 mm and 0.7 - 1.0 grams.
The designation or denomination of a coin is based on the usual size for the type. The coins were handmade and the same type can vary significantly in size. Some individual coins that are larger or smaller than usual for their type will be outside the parameters above.
Also see Size Scales.