There is no exact definition per se. However here are some guidelines:

People have told me it’s loans with FICO scores of 620 or less. Others have told me it’s 660 or less. But I’ve seen loan matrices that count loans as subprime with FICOs above 660 but loan-to-value ratios of 95 percent plus, or with low documentation, or both. People outside the industry might count those as subprime and people in the industry are more likely to call them Alt-A.