what is a dialect?
There is a tremendous amount of different views of what constitutes a dialect, up to the point that many people, especially linguists, avoid using the word altogether, and not without reason. It is impossible to formulate a definition that satisfies everybody, all the more since it will always be subjective and arbitrary. So you will not find a definitive answer to the question “What is a dialect?” here. Instead, you will find on this page some of the countless answers given throughout history by people who are much smarter than I am. On a regular basis, I will add new definitions here, usually accompanied by a very short note on the author and his view on what constitutes a dialect.
For those who can’t get enough of dialect definitions: you can download here a 101-page anthology covering the premodern period up to about 1800. The definitions are offered there in their original language (with standardized Latin orthography).
Quintilian (ca. AD 35-100)
This law is more difficult to formulate for the Greeks, because they have several varieties of speech, which they call dialects, and what is wrong in one is sometimes correct in another.
The Roman rhetorician Quintilian did not exactly formulate a precise definition of ‘dialect,’ which he likely used in the original Greek form diálektos (διάλεκτος). Yet, in his handbook for the orator, he did give a glimpse of how he conceived of it when commenting upon accentuation in Latin and Greek. Dialects were ‘kinds of speaking’ (loquendi genera in the original Latin) with their own properties and grammatical features, which were unacceptable in other dialects. For Quintilian it was obvious that the Greek dialects equaled the literary dialects Aeolic, Attic, Doric, and Ionic, which he compared to different literary registers in Latin.
English translation taken from Loeb. The original Latin is at Quintilian, Institutio oratoria 1.5.29: Cuius difficilior apud Graecos observatio est, quia plura illis loquendi genera, quas dialectus [= διαλέκτους] vocant, et quod alias vitiosum, interim alias rectum est.