At Risk Rules
Tax laws limiting the amount of losses an investor (usually a limited partner) can claim. Only the amount actually at risk can be deducted.
Investopedia Commentary
For example, if you have $20,000 at risk in an investment which generates $5000 in tax losses a year, the losses may only be used for 4 years (or until the investor puts more money in).
See also: Alternative Minimum Tax, Income Tax, Risk