The fairly uniform spoken Greek that gradually replaced the local dialects after the breakdown of old political barriers and the establishment of Alexander’s empire in the 4th century bce is known as the Koine (hē koinē dialektos ‘the common language’), or “Hellenistic Greek.” Attic, by virtue of the undiminished cultural and commercial predominance of Athens, provided its basis, but, as the medium of communication throughout the new urban centres of Egypt, Syria, and Asia Minor, it absorbed numerous non-Attic elements and underwent some degree of grammatical simplification. Numerous inscriptions enable scholars to trace its triumphant progress at the expense of the old dialects, at least as the language of business and administration, although some rural dialects are reported to have survived as late as the 2nd century ce. Other sources of information for the Koine are the translation of the Septuagint made in the 3rd century bce for the use of the Hellenized Jewish community of Alexandria, the New Testament, and the writings of a few people (e.g., the historian Polybius and the philosopher Epictetus) who preferred it to Attic. As the everyday colloquial language of urban Egypt, it may be studied in papyri going back to the 4th century bce. The Koine may be dated very crudely from the period of Alexander’s conquests in the 4th century bce to approximately the reign of Justinian in the 6th century ce.
The Koine replaced the Attic tt with the ss characteristic of Ionic and other dialects (e.g., glōssa for glōtta ‘tongue’) at an early date, but its main phonological characteristic is the gradual simplification of the rich vowel system of Classical Greek. Ancient closed and open long /ē/ (ει and η) and /i/ (ι) merged as /i/, and /ai/ (αι) monophthongized to /e/; /oi/ (οι) monophthongized to /ü/, thus merging with simple /ü/ (υ) (pronounced as French tu). The second element of /au/ (αυ) and /eu/ (ευ) was changed to /v/ or /f/ depending on the voicing of the following consonant (compare Ancient auge ‘light, dawn,’ autos ‘he’ to Modern avghí, aftós). Classical /ph th kh/ (pronounced as in English pin, tin, kin) acquired fricative articulations as in fin, thin, and the final element of Scottish loch (or German Buch); /b d g/ became the voiced fricatives /v dh/ (as in that), /gh/ (as in Spanish fuego).
Other parts of the grammar also began to move in the direction of Modern Greek in this period. Nouns in consonant stems began to acquire the endings of the -a declension; e.g., thygatēr, ‘daughter,’ accusative thygatera, was remodeled after items such as khōra, khōran ‘country.’ The dual number was lost in nouns, verbs, and adjectives, as was the optative mood (expressing wish or desire) of verbs. Confusion arose between the perfect and aorist tense forms, leading to the loss of one or the other (the former in most verbs).
In vocabulary there were numerous borrowings from non-Attic dialects, and some Attic words acquired new meanings; thus, opsaria ‘fish’ and brechei ‘it rains’ for Classical Greek ichthyes and hyei both occur in the New Testament (compare Modern Greek psárya, vrékhi).
This gradual divergence from the language of Plato and Demosthenes was viewed as a species of linguistic decadence by an influential school known as the Atticists, who unceasingly castigated the use of Koine forms by writers. It was thus that the rift developed between the everyday spoken language and an archaizing, specifically written language. It became fashionable to publish manuals of “good usage” in which the Attic equivalents of Koine innovations were recommended as models for the student’s imitation.