event descriptor
Technical computing term for a data structure or identifier that describes an event, used in contexts such as GUI programming, event-driven systems, or hardware event handling.
In this API, event descriptors are used to identify the type of event that occurred.
指定子 means 'specifier' or 'descriptor' and is used in various technical compounds; イベント指定子 is one such compound.
Compound of イベント (event) and 指定子 (specifier/descriptor), a direct calque from English 'event descriptor' used in computing.