One of the mysteries of the English language finally explained.
Used to introduce an explanation of a word or phrase: that is to say.
Old English; earliest use found in Bede's Ecclesiastical History. From classical Latin id est ‘that is’, used in works written in Latin to introduce an explanation of a word or phrase, ‘that is to say’ from id it + est, 3rd singular present indicative of esse to be.
id est/(ˌ)ɪd ˈɛst/
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