The Phenomenon of diglossia is. regarded as one of the most significant linguistic Phenomena in the Arabic language. It refers to the existence of two distinct linguistic varieties within The same speech Community namely Classical Cot standard) Arabic and Colloquial Arabic. This Phenomenon is Clearly dbserved in daily communication, Where speakers alternate between These two varieties according To The Communicative Context. The Arabic language has attracted the attention of many linguists due to the widespread nature of this phenomenon.Numerous scholars have examined diglossia and discussed its causes and its effects on the Arabic language. This phenomenon is considered and linguistic issue that has been associated with the linguistic reality of Arabic sinsce early times-Diglossia emerged as a result of the distinction between the language of The Holy Quran and the spoken language of The People. This distinction led to the development of the People. This distinction led to the development of Iwolinguistic levels, eash characterized by specif functions and domains of use. Consequently, diglossia has influenced language use in several fields, including education, writing, and Communication.This research aims to Clarify the Concept of diglossia, identify its causes. and effects, and highlight its impact on The Arabic language. Keywords: Synonymy – Language – Terminology – Ancients – Moderns – Affirmation – Denial.