IPA: //koʊˈpʌblɪʃər//
KK: /ˈkoʊˌpʌblɪʃər/
A person or company that shares the responsibility of publishing a work with one or more other publishers.
The book was released by a copublisher, ensuring a wider distribution.
Copublisher → It is formed from "co-" (meaning together) and "publisher" (from Old French *publier*, meaning to make public). The word "copublisher" refers to someone who publishes something together with another person or entity.
Think of 'together' ('co-') and 'making public' ('publisher') — that's why a copublisher is someone who publishes something alongside another.