From the IRS website: https://www.irs.gov/newsroom/irs-issues-guidance-on-changes-to-excess-business-and-net-operating-los...
"An excess business loss is the amount by which the total deductions from all trades or businesses exceed a taxpayer’s total gross income and gains from those trades or businesses, plus $250,000, or $500,000 for a joint return.
Excess business losses that are disallowed are treated as a net operating loss carryover to the following taxable year.
See Form 461 and instructions, available soon, for details."
