3. The term ‘portent’ (portentum) is said to be derived from foreshadowing (portendere), that is, from ‘showing beforehand’ (praeostendere). ‘Signs’ (ostentum), because they seem to show (ostendere) a future event. Prodigies (prodigium) are so called, because they ‘speak hereafter’ (porro dicere), that is, they predict the future. But omens (monstrum) derive their name from admonition (monitus), because in giving a sign they indicate (demonstrare) something, or else because they instantly show (monstrare) what may appear; and this is its proper meaning, even though it has frequently been corrupted by the improper use of writers.
The Etymologies of Isidore of Seville
Book xi The human being and portents