When Are Age Spots Diagnosed As Melasma?

At what point are "age spots" considered "melasma"? Is it the size of the spot that puts it in the latter category? Or percentage of the skin? I've always called my dark spots "age spots".

My doctor just refers to it as "hyperpigmentation" but I have to wonder if it's really "melasma"? Is there a standard?

Help! And no, it really doesn't matter to me what you call it, I just want to get rid of it -- effectively. Thanks.