Wouldn't that be just a word? I mean, the person asking likely recognizes that it IS a word, s/he just doesn't know what the word means. There IS likely a special term (compound word? Is it a compound if the words are written sperately but are used together to convey on information?), but I'd just use word.
Sound, maybe the sound that happens when somebody says credit card. If the person asking doesn't realize it's a word, but did realize that credit and card go together, wouldn't that just be a sound?
I mean, if she recognized that credit card has some meaning, the person asks for the meaning of the words credit and card, used together (or creditcard if the person understood it that way, which doesn't really change the meaning). If the person doesn't realize credit card is a word, the person would just have heard a sound, the sound of credit card.