The definition of repeat buyer is simply a buyer who generates multiple transactions with you. Sometimes, that comes about through multiple product sales — the buyer purchases one product, and then later purchases a different product (or hopefully multiple additional products).

A continuity program is even better — it is multiple transactions for the same product. In other words, sell it once and keep billing for it over and over again.

Now, in looking at my definitions above, I see a bit of a hole (hence your question). Perhaps I should add another category called “Continuity Buyer?” I still prefer the term buyer because I believe that a buying decision is made every time a transaction takes place. “Customer” has a past-tense feel to it, but “Buyer” is someone who is active.