Nomina sacra
A small group of words referring to God (i.e., "holy names") that are represented by conventional abbreviations by contraction.
"Holy names" – a small group of words referring to God that are represented by conventional abbreviations by contraction, normally in litterae notabiliores. Examples are DS (Deus), DNS (Dominus), SPS SCS (Spiritus Sanctus). Forms of Jesus' name are normally given in Greek letters: e.g. IHS or IHC (iota-eta-sigma) for Iesus and XPS (chi-rho-sigma) for Christus.
Paap, A.H.R.E. Nomina Sacra in the Greek Papyri of the First Five Centuries A.D.: The Sources and Some Deductions. Papyrologica Lugduno-Batava 8. Leiden, 1959.
Traube, Ludwig. Nomina Sacra: Versuch einer Geschichte der christlichen Kürzung. Munich, 1907.