A catacomb is a subterranean excavation for the interment of the dead or a burial vault. In this sense the word catacomb has gained universal acceptance, and has found a place in most modern languages. The original term, catacumbae, however, had no connection with sepulture, but was simply the name of a particular locality in the environs of Rome. It was derived from the Greek and had reference to the natural configuration of the ground.

In the district that bore this designation, lying close to the Appian Way, the basilica of San Sebastiano was erected, and the extensive burial vaults beneath that church, in which according to tradition, the bodies of the apostles St. Peter and St. Paul rested for a year and seven months previous to their removal to the basilicas which bear their names were in very early times, called from it coemeterium ad catacumbas, or catacumbas alone. 

From the celebrity of this cemetery as an object of pilgrimage its name became extensively known, and in entire forgetfulness of the origin of the word, catacumbae came to be regarded as a generic appellation for all burial places of the same kind. This extension of the term to Christian burial vaults generally dates from the 9th century, and obtained gradual currency through the Christian world. The original designation of these places of sepulture is crypta or coemeterium.