It's a loan with a guaranteed atomicity in repaying.

A single atomic transaction does:

1) Borrow $80M 2) Use $80M however you want 3) Return $80M + loan fees (e.g. on Aave this would be 0.09%)

The lender is algorithmically guaranteed to get the money back and the borrower can potentially take advantage of large scale transient opportunities, or...just wreak havoc on a poorly secured system.

More info here: https://docs.aave.com/developers/guides/flash-loans