Synonyms are different words that present the same meaning. When we look for the meaning of cave, we find "a large underground chamber, typically of natural origin, in a hillside or cliff." As soon as we look for the meaning of cavern, we find it described as "a cave." Since they refer to the same thing, dictionaries use one as the immediate explanation for the other.
The same happens when we look up the word tune. Dictionaries offer synonyms for it and, among the very first ones, we find the word song. They both refer to a composition of words and music.
