There is not one definition, if you hear “4K” you can use the context of the conversation to determine if they’re talking about the consumer 4K UHD format or cinematic 4K
And I don’t want a definition for “4k DCI” or “4k UHD” … just a formally accepted definition of “4k” (in the context of a display resolution). We can all agree that it colloquially means the number 4000, I hope.