Ask HN: How do you define the word “protocol”?

I an having trouble finding a definition for this word, which is super weird, as it is ubiquitous. Some have ranged from "procedure to call subprogram" to the broad "a system of communication".

How would you describe what a protocol is to someone not in the industry?

I know what a protocol is, but I want to know if there is a set of requirements or commonalities between all protocols and what are the "requirements".