c - Is it a definition or a declaration?

Your understanding is correct. Your code example is a
declaration of a type. In 'C' you can declare a type and immediately use it to define a
variable.

So your example is a pure
declaration.

And here is an example of declaration+variable
definition:

struct foo {
 char
            name[10];
 char title[10];
 int salary;
}
            var;