In Roman religion, Terminus was the god who protected boundary markers; his name was the Latin word for such a marker. Sacrifices were performed to sanctify each boundary stone, and landowners celebrated a festival called the "Terminalia" in Terminus' honor each year on February 23. The Temple of Jupiter Optimus Maximus on the Capitoline Hill was thought to have been built over a shrine to Terminus, and he was occasionally identified as an aspect of Jupiter under the name "Jupiter Terminalis".
Ancient writers believed that the worship of Terminus had been introduced to Rome during the reign of the first king Romulus (traditionally 753–717 BC) or his successor Numa (717–673 BC). Modern scholars have variously seen it as the survival of an early animistic reverence for the power inherent in the boundary marker, or as the Roman development of proto-Indo-European belief in a god concerned with the division of property.
The name of the god Terminus was the Latin word for a boundary stone, and his worship as recorded in the late Republic and Empire centred on this stone, with which the god could be identified. Siculus Flaccus, a writer on land surveying, records the ritual by which the stone was sanctified: the bones, ashes, and blood of a sacrificial victim, along with crops, honeycombs, and wine, were placed into a hole at a point where estates converged, and the stone was driven in on top.
On February 23 annually, a festival called the Terminalia was celebrated in Terminus' honor, involving practices which can be regarded as a reflection or "yearly renewal" of this foundational ritual. Neighboring families would garland their respective sides of the marker and make offerings to Terminus at an altar—Ovid identifies these, again, as crops, honeycombs, and wine. The marker itself would be drenched in the blood of a sacrificed lamb or pig. There followed a communal feast and hymns in praise of Terminus.
These rites were practised by private landowners, but there were also related public ceremonies. Ovid refers to the sacrifice of a sheep on the day of the Terminalia at the sixth milestone from Rome along the Via Laurentina; it is likely this was thought to have marked the boundary between the early Romans and their neighbors in Laurentum.
Also, a stone or altar of Terminus was located in the Temple of Jupiter Optimus Maximus on Rome's Capitoline Hill. Because of a belief that this stone had to be exposed to the sky, there was a small hole in the ceiling directly above it. On occasion Terminus' association with Jupiter extended to regarding Terminus as an aspect of that god; Dionysius of Halicarnassus refers to "Jupiter Terminalis", and one inscription names a god "Jupiter Ter."
There is some evidence that Terminus' associations could extend from property boundaries to limits more generally. Under the Republican calendar, when the intercalary month Mercedonius was added to a year, it was placed after February 23 or February 24, and some ancient writers believed that the Terminalia on February 23 had once been the end of the year. Diocletian's decision in 303 AD to initiate his persecution of Christians on February 23 has been seen as an attempt at enlisting Terminus "to put a limit to the progress of Christianity".