This is not an easy definition. It appears to be an easy definition which is one of the biggest problems we have to face in communication. Outstanding minds have wrestled with this problem for years. This is what George Bernard Shaw had to say about it.
“The single biggest problem in communication is the illusion that it has taken place.”
That assumes that there is some sort of expectation involved in the process. "I said it and I expected them to . . ." and here you can fill in your own thought. When you ask for something concrete like "Please pass the salt," and you get the salt you have evidence that something took place: communication. What about areas that are distant or more abstract and what you expected doesn't happen. Then the assumption is that communication didn't occur.
Shaw's observation is very evident every day in most of our lives. We assume that the what we have said is understood in the way we expected to be understood and that certain specific result will occur. When they don't we assume that there has been a failure to communicate.
If you have power or authority and you "know you are perfectly clear" in your communication then you risk the assumption that communication has taken place. That can lead to fights, hurt feelings, war, and lots of negative things. Be cautious about assumptions and don't allow illusions to mess up your life.