The word testament in the expression "New Testament" refers to a new covenant that Christians believe completes or fulfils the Mosaic covenant (the old covenant) that Yahweh made with the people of Israel on Mount Sinai through Moses, described in the books of the Old Testament. Christians traditionally view this new covenant as being prophesied in (Jeremiah 31–34).

The word covenant means 'agreement' (from Latin con-venio 'to agree' lit. 'to come together'): the use of the word testament, which describes the different idea of written instructions for inheritance after death, to refer to the covenant with Israel in the Old Testament, is foreign to the original Hebrew brit (בְּרִית) describing it, which only means 'alliance, covenant, pact' and never 'inheritance instructions after death'. This use comes from the Latin testamentum 'will (left after death)', a literal translation of Greek διαθήκη 'will (left after death)', which is the word used to translate Hebrew brit in the Septuagint (for example in Κιβωτός της Διαθήκης for arón habrít (אֲרוֹן הַבְּרִית) 'Ark of the Covenant').

The choice of this word διαθήκη, by the Jewish translators of the Septuagint in Alexandria in the 3rd and 2nd century BCE, has been understood in Christian theology to imply a reinterpreted view of the Old Testament covenant with Israel as possessing characteristics of a 'will left after death' (the death of Jesus) and has generated considerable attention from biblical scholars and theologians: in contrast to the Jewish usage where brit was the usual Hebrew word used to refer to pacts, alliances and covenants in general, like a common pact between two individuals, and to the one between God and Israel in particular, in the Greek world διαθήκη was virtually never used to refer to an alliance or covenant (one exception is noted in a passage from Aristophanes) and referred instead to a will left after the death of a person. There is scholarly debate as to the reason why the translators of the Septuagint chose the term διαθήκη to translate Hebrew brit, instead of another Greek word generally used to refer to an alliance or covenant.

The use of the phrase New Testament (Ἡ Καινὴ Διαθήκη) to describe a collection of first- and second-century Christian Greek scriptures can be traced back to Tertullian in his work Against Praxeas.

Jackson, Bernard S. (2013). "Why the Name New Testament?". Melilah: Manchester Journal of Jewish Studies. 9 (1): 50–100. doi:10.31826/mjj-2013-090104. http://static1.1.sqspcdn.com/static/f/784513/23609902/1380577434807/3.pdf?token=8ZveuYFof7uMu8UwoCMRepxQitY%3D

Norton, F.O., A Lexicographical and Historical Study of DIAΘHKH from the Earliest Times to the End of the Classical Period (Chicago: University of Chicago Press, 1908

Behm, J., “διαθήκη” (sections B-D), in Theological Dictionary of the New Testament, ed. G. Kittel and G. Friedrich, trans. G.W. Bromiley (Grand Rapids MI: Eerdmans, 1965), II.124–34.