In English, we care about whether something is general or specific. That's why we use articles like "a" and "the":
I saw a dog.
I saw the dog.
The first sentence is general. It's about a dog which the listener doesn't know about. The second sentence is specific. It's about a certain dog that the listener has seen or heard about.
But sometimes "the" isn't strong enough by itself. One way to make it stronger is with the word "specific":
I've seen a dog that looks like the one you described. I don't know if it was the specific dog you were referring to, though.