The term "catacombs" used as the name for a network of subterranean burial grounds has been widely accepted as being derived from the Greek kata kumbas (=Latin, ad catacumbas, or "near the hollows").  The expression may have been originally a topographical reference to the site of a first-century pagan cemetery between the second and third milestones of the Appian Way.  The cemetery lay in a deep hollow, a former quarry, that had been left partly open to the sky and in part dug underground as a series of tunnels, well suited for various types of burials (1).  In the second half of the third century, when, according to P. Testini, the site could have been "penetrated" by Christianity (2), it became the center for the veneration of Peter and Paul.  In the fourth century, a church was built over the cemetery as a memorial or sanctuary for the cult of the two saints, the Basilica Apostolorum.

It is believed that the site was associated with the apostles because, during the persecutions of the Christians by the emperor Valerian in 257, the saints' relics were taken from the necropoleis where they had been buried after their deaths and reburied at the via Appia site for safety.  There are records of a celebration of their cult there on June 28, 258.  In the fourth century, their bones were returned to the original graves (3).

When the remains of St. Sebastian were buried in the cemetery, the church was renamed for him.  The cemetery under San Sebastiano was one of perhaps four that continued to be visited by pilgrims into the Middle Ages, and the special sanctity of this burial site may be the reason that the term "catacomb" came to be used for all such subterranean cemeteries, private or communal.

The names of the Roman Jewish catacombs and a number of the Christian sites are, like the word catacomb itself, toponomical, derived from street locations or from distinguishing landmarks.  Other catacombs, such as those of Priscilla, Domitilla, and Praetextatus, were named for affluent Roman families who evidently had made land available to early Christians for burial places from which the catacombs were later developed.  After the onset of the fourth century, many Christian burial grounds received the names of martyrs entombed within them (4).  Martyr cults were particularly popular from the fourth century on, reminiscent of the hero cults of more ancient times.  The building of martyrial shrines and cemetery basilicas led to an increase in the density of the burials in the Christian catacombs because of the desire of the faithful to be buried close to the venerated tombs.  

The devotion to the memory of the Christian martyrs, as well as the impression made by the catacombs on the sensibilities of the Christians in late fourth-century Rome, were expressed in the words of Jerome (5):

When I was a youth in Roe, studying liberal arts, it was my custom on Sundays... to visit the sepulchers of the apostles and martyrs.  And often did I enter the crypts, dug in the depths of the earth, with their walls on either side lined with the bodies of the dead, where the darkness was such that it almost seemed as if the psalmist's words were fulfilled: 'Let them go down quick (alive) into hell.' Here and there a ray of light... filtering down as in a funnel relieves the horror of the darkness.  But, again, cautiously moving forward, the impenetrable night engulfed me, and I was reminded of the words of Vergil: 'Everywhere, dread fills the soul; the very silence dismays' (6).

1. P. Pergola, Le catacombe Romane, Rome: Carocci, 1998, pp. 22, 181-185.  A theory of Father Ferrua was that the site might have received its name from a signboard of a nearby inn called Ad Cumbas ("near the small boats") or a relief depicting two or more small boats: A. Ferrua, Guide to the Basilica and Catacomb of Saint Sebastian, 1978, p. 7.
2. Testini, Cimiteri cristiani, pp. 216-218, 221.
3. M. Guarducci, Peter, pp. 25-26; Pergola, Le catacombe romane, pp. 184-185; J. Stevenson, The Catacombs: Life and Death in Early Christianity.  Nashville: T. Nelson, 1985, p. 32.
4. An exception to the above described origins of catacomb names is the Catacomb of Callisto, which was named for the overseer of the cemetery: Stevenson, The Catacombs, p. 25.
5. The secretary of Pope Damasus, Jerome (340-420) was a Latin and Hebrew scholar whose translation of the Bible into latin is known as the Vulgate.
6. J. N. D. Kelly, Jerome, His Life, Writings, and Controversies, New York: Harper & Row, p. 22; Ps. 55:15; Aeneid 2, 755.