Wisdom from the Early Stoics, Zeno of Citium 32

There are five excellences of speech—pure Greek, lucidity, conciseness, appropriateness, distinction. 

By good Greek is meant language faultless in point of grammar and free from careless vulgarity. 

Lucidity is a style which presents the thought in a way easily understood; conciseness a style that employs no more words than are necessary for setting forth the subject in hand; appropriateness lies in a style akin to the subject; distinction in the avoidance of colloquialism. 

Among vices of style barbarism is violation of the usage of Greeks of good standing; while there is solecism when the sentence has an incongruous construction.

—Diogenes Laërtius, 7.59