1. a list or collection of proper names.
2. a list or collection of specialized terms, as those used in a particular field or subject area.
A quick look at any onomasticon shows how many more Greek names begin with alpha than with any other letter.
— Edited by Benjamin Acosta-Hughes, Elizabeth Kosmetatou and Manuel Baumbach , Labored in Papyrus Leaves: Perspectives on an Epigram Collection Attributed to Posidippus , 2004
We bring this up because today happens to be the 162nd birthday of Mr. Roget’s compendium of synonyms. Yes, on this date in 1852, the first edition of Peter Roget’s onomasticon was published.
— Greg Mandel , “Tuesday’s Edge: Have you seen today’s Butt?” The Oregonian , 2014
Onomasticon entered English in the early 1700s from the Greek onomastikòn meaning “vocabulary arranged by subjects.”