C#: What is the difference between delegate and event?

1// Delegate
2public delegate void NotifyHandler(string message);
3
4public class Publisher {
5    public NotifyHandler OnNotify;
6
7    public void DoSomething() {
8        OnNotify?.Invoke("Done!");
9    }
10}
11
12// Event
13public class Publisher {
14    public event NotifyHandler OnNotify;
15
16    public void DoSomething() {
17        OnNotify?.Invoke("Done!");  // OK
18    }
19}