Alt-A

What Does Alt-A Mean?
A classification of mortgages where the risk profile falls between prime and subprime. The borrowers behind these mortgages will typically have clean credit histories, but the mortgage itself will generally have some issues that increase its risk profile. These issues include higher loan-to-value and debt-to-income ratios or inadequate documentation of the borrower's income.

Investopedia explains Alt-A
These types of loans are attractive to lenders because the rates are higher than rates on prime classified mortgages, but they are still backed by borrowers with stronger credit ratings than subprime borrowers. However, with the higher rates comes additional risk for lenders because there is a lack of documentation - including limited proof of the borrower's income.