NOL is a Net Operating Loss.
From IRS Publication 536 (2019), Net Operating Losses (NOLs) for Individuals, Estates, and Trusts:
"If your deductions for the year are more than your income for the year, you may have a net operating loss (NOL). An NOL year is the year in which an NOL occurs. You can use an NOL by deducting it from your income in another year or years."
