388 AD Theodosius I 4.40g, 20.26mm
This is a Solidus - the denomination introduced by Constantine the great which would continue to be struck with very little variation in weight or fineness for nearly another thousand years as it became the flagship coin of the Byzantine empire.
Your specimen was minted at Trier, between 388 and 392 A.D. The "COM" in the exergue refers to it being pure gold - the mint designation is the T - R to either side of the two characters on the reverse - who are meant to be 2 co-regnal emperors - the basic design goes back to the time of Valentinian I and Valens, but continued until, at this time, it was supposed to be Valentinian II and Theodosius I as the 2 senior Augusti, there was however (throughout the era) a 3rd and even, at times, a 4th emperor.
The obverse legend is D N THEODOSIVS P F AVG - with the normal diademed draped and cuirassed bust right.
The reverse is VICTORIA AVGG (the 2 "G's" abbreviating "Augustorum" referring to multiple emperors) and shows the 2 emperors, presumably Valentinian II and Theodosius I seated facing, holding a globe between them, the top half of Victory between and spreading her wings above them.
RIC IX Trier 90b.