To understand the nouns, let's focus on the verbs first.
declare - to announce officially; proclaim
define - to show or describe (someone or something) clearly and completely
So, when you declare something, you just tell what it is.
// declaration
int sum(int, int);
This line declares a C function called sum that takes two arguments of type int and returns an int. However, you can't use it yet.
When you provide how it actually works, that's the definition of it.
// definition
int sum(int x, int y)
{
return x + y;
}