Following the continuing crises that characterized much of the third century, Gaius Aurelius Valerius Diocletianus. who ruled during most of his reign in conjunction with Maximian, stabilized the empire politically and militarily. The sharing of power later evolved into the Tetrarchy, to include Galerius and Constantius Chlorus in the governance of Rome's most distant provinces.
Diocletian has gone down in history as being responsible for the last widespread persecutions of Christians.
When Diocletian abdicated due to health issues, and Maximian leaving with him, the hope and intent was for an orderly transfer of their offices. Instead, their departures ushered in another period of civil conflict and unrest as various claimants vied for their positions. Ultimately, Constantius' son Constantine (the Great) would emerge triumphant to the exclusion of all others.
This coin is roughly 28 mm across and weighs just under 11 grams. The TSA mint mark indicates it was struck in. Thessalonica, and it would appear to be a variant of RIC VI 19.

