Koine Greek represents what could be considered to be the final stage of what can be termed as "Old Greek", a nebulous concept spanning centuries, comprising the stages of Mycenaean Greek, Archaic Greek, Classical Greek, and finally Koine Greek.
Koine Greek comes from the word κοινη, meaning "common". Koine arose from dialect leveling that arose in the Hellenistic period. Koine represents the demotic register of the Greek language, in contrast to the formalized and literary register of Attic Greek that served as the language of literature and poetry
Perhaps most famously, Koine Greek is the form of thr Greek language that was used in the New Testament of the Bible, leading to a common designation of Koine as "New Testament Greek".