Nomina sacra (singular: nomen sacrum) means "sacred names" in Latin, and can be used to refer to traditions of abbreviated writing of several frequently occurring divine names or titles in early Holy Scripture, used in Greek, Latin, and Coptic manuscripts. Bruce Metzger's book Manuscripts of the Greek Bible lists 15 such expressions from Greek papyri: the Greek counterparts of God, Lord, Jesus, Christ, Son, Spirit, David, cross, Mother, Father, Israel, Savior, Man, Jerusalem, and Heaven. The nomen sacrum for mother did not appear until the 4th century CE, but all other Nomina Sacra have been found in Greek manuscripts from the 1st - 3rd Centuries CE. The contractions were indicated with overlines.
There has been a dispute about the nature of Nomina sacra, whether they represent a mere shorthand or these overlined words indeed bear a sacred meaning.
Starting sometime in the 1st Century CE (exact date unknown), the nomina sacra were sometimes shortened by contraction in Christian inscriptions, resulting in sequences of Greek letters such as IH (iota-eta), IC (iota-sigma), or IHC (iota-eta-sigma) for Jesus (Greek Iēsous), and XC (chi-sigma), XP (chi-ro) and XPC (chi-rho-sigma) for Christ (Greek χριστος/Christos). Here "C" represents the "lunate" form of Greek sigma; sigma could also be transcribed into the Latin alphabet by sound, giving IHS and XPS.
This tradition is also observed in Old Nubian and Cyrillic manuscripts. See titlo.
Read more about Nomina Sacra: List of Greek Nomina Sacra, New Testament Greek Manuscripts Containing Nomina Sacra (100 AD - 300 AD)